Malkuth is the Cinderella of the sephiroth. It is the sephira most often ignored by beginners, the sephira most often glossed over in Kabbalistic texts, and it is not only the most immediate of the sephira but it is also the most complex, and for sheer inscrutability it rivals Kether - indeed, there is a Kabbalistic aphorism that "Kether is Malkuth, and Malkuth is in Kether, but after another manner".
The word Malkuth means "Kingdom", and the sephira is the culmination of a process of emanation whereby the creative power of the Godhead is progressively structured and defined as it moves down the Tree and arrives in a completed form in Malkuth. Malkuth is the sphere of matter, substance, the real, physical world. In the least compromising versions of materialist philosophy (e.g. Hobbes) there is nothing beyond physical matter, and from that viewpoint the Tree of Life beyond Malkuth does not exist: our feelings of identity and self-consciousness are nothing more than a by-product of chemical reactions in the brain, and the mind is a complex automata which suffers from the disease of metaphysical delusions. Kabbalah is not a materialist model of reality, but when we examine Malkuth by itself we find ourselves immersed in matter, and it is natural to think in terms of physics, chemistry and molecular biology. The natural sciences provide the most accurate models of matter and the physical world that we have, and it would be foolishness of the first order to imagine that Kabbalah can provide better explanations of the nature of matter on the basis of a study of the text of the Old Testament. Not that I under-rate the intuition which has gone into the making of Kabbalah over the centuries, but for practical purposes the average university science graduate knows (much) more about the material stuff of the world than medieval Kabbalists, and a grounding in modern physics is as good a way to approach Malkuth as any other.
For those who are not comfortable with physics there are alternative, more traditional ways of approaching Malkuth. The magical image of Malkuth is that of a young woman crowned and throned. The woman is Malkah, the Queen, Kallah, the Bride. She is the inferior mother, a reflection and realisation of the superior mother Binah. She is the Queen who inhabits the Kingdom, and the Bride of the Microprosopus. She is Gaia, Mother Earth, but of course she is not only the substance of this world; she is the body of the entire physical universe.
Some care is required when assigning Mother/Earth goddesses to Malkuth, because some of them correspond more closely to the superior mother Binah. There is a close and deep connection between Malkuth and Binah which results in the two sephiroth sharing similar correspondences, and one of the oldest Kabbalistic texts  has this to say about Malkuth:
"The title of the tenth path [Malkuth] is the Resplendent Intelligence. It is called this because it is exalted above every head from where it sits upon the throne of Binah. It illuminates the numinosity of all lights and causes to emanate the Power of the archetype of countenances or forms."
One of the titles of Binah is Khorsia, or Throne, and the image which this text provides is that Binah provides the framework upon which Malkuth sits. We will return to this later. Binah contains the potential of form in the abstract, while Malkuth is is the fullest realisation of form, and both sephiroth share the correspondences of heaviness, limitation, finiteness, inertia, avarice, silence, and death.
The female quality of Malkuth is often identified with the Shekhinah, the female spirit of God in the creation, and Kabbalistic literature makes much of the (carnal) relationship of God and the Shekhinah. Waite  mentions that the relationship between God and Shekhinah is mirrored in the relationship between man and woman, and provides a great deal of information on both the Shekhinah and what he quaintly calls "The Mystery of Sex". After the exile of the Jews from Spain in 1492, Kabbalists identified their own plight with the fate of the Shekhinah, and she is pictured as being cast out into matter in much the same way as the Gnostics pictured Sophia, the outcast divine wisdom. The doctrine of the Shekhinah within Kabbalah and within Judaism as a whole is complex and it is something I don't feel competent to comment further on; more information can be found in  & .
Malkuth is the sphere of the physical elements and Kabbalists still use the four-fold scheme which dates back at least as far as Empedocles and probably the Ark. The four elements correspond to four readily-observable states of matter:
In addition it is not uncommon to include a fifth element so rarified and arcane that most people (self included) are pushed to say what it is; the fifth element is aethyr (or ether) and is sometimes called spirit.
The amount of material written about the elements is enormous, and rather than reproduce in bulk what is relatively well-known I will provide a rough outline so that those readers who aren't familiar with Kabbalah will realise I am talking about approximately the same thing as they have seen before. A detailed description of the traditional medieval view of the four elements can be found in "The Magus" . The hierarchy of elemental powers can be found in "777"  and in Golden Dawn material  - I have summarised a few useful items below:
Element Fire Air Water Earth God Name Elohim Jehovah Eheieh Agla Archangel Michael Raphael Gabriel Uriel King Djin Paralda Nichsa Ghob Elemental Salamanders Sylphs Undines Gnomes
It amused me to notice that the section on the elemental kingdoms in Farrar's "What Witches Do"  had been taken by Alex Saunders lock, stock and barrel from traditional Kabbalistic and CM sources.
The elements in Malkuth are arranged as follows:
South Fire East Zenith Aethyr+ West Air Nadir Aethyr- Water North Earth
I have rotated the cardinal points through 180 degrees from their customary directions so that it is easier to see how the elements fit on the lower face of the Tree of Life:
Tiphereth Fire Hod Yesod Netzach Air Aethyr Water Malkuth Earth
It is important to distinguish between the elements in Malkuth, where we are talking about real substance (the water in your body, the breath in your lungs), and the elements on the Tree, where we are using traditional correspondences associated with the elements, e.g.:
Aethyr or Spirit is enigmatic, and I tend to think of it in terms of the forces which bind matter together. It is almost certainly a coincidence (but nevertheless interesting) that there are four fundamental forces - gravitational, electromagnetic, weak nuclear & strong nuclear - known to date, and current belief is that they can be unified into one fundamental force. On a slightly more arcane tack, Barret  has this to say about Aethyr:
"Now seeing that the soul is the essential form, intelligible and uncorruptible, and is the first mover of the body, and is moved itself; but that the body, or matter, is of itself unable and unfit for motion, and does very much degenerate from the soul, it appears that there is a need of a more excellent medium:- now such a medium is conceived to be the spirit of the world, or that which some call a quintessence; because it is not from the four elements, but a certain first thing, having its being above and beside them. There is, therefore, such a kind of medium required to be, by which celestial souls [e.g. forms] may be joined to gross bodies, and bestow upon them wonderful gifts. This spirit is in the same manner, in the body of the world, as our spirit is in our bodies; for as the powers of our soul are communicated to the members of the body by the medium of the spirit, so also the virtue of the soul of the world is diffused, throughout all things, by the medium of the universal spirit; for there is nothing to be found in the whole world that hath not a spark of the virtue thereof."
Aethyr underpins the elements like a foundation and its attribution to Yesod should be obvious, particularly as it forms the linking role between the ideoplastic world of "the Astral Light"  and the material world. Aethyr is often thought to come in two flavours - positive Aethyr, which binds, and negative Aethyr, which unbinds. Negative Aethyr is a bit like the Universal Solvent, and requires as much care in handling ;-}
Working with the physical elements in Malkuth is one of the most important areas of applied magic, dealing as it does with the basic constituents of the real world. The physical elements are tangible and can be experience in a very direct way through recreations such as caving, diving, parachuting or firewalking; they bite back in a suitably humbling way, and they provide CMs with an opportunity to join the neo-pagans in the great outdoors. Our bodies themselves are made from physical stuff, and there are many Raja Yoga-like exercises which can be carried out using the elements as a basis for work on the body. If you can stand his manic intensity (Exercise 1: boil an egg by force of will) then Bardon  is full of good ideas.
Malkuth is often associated with various kinds of intrinsic evil, and to understand this attitude (which I do not share) it is necessary to confront the same question as thirteenth century Kabbalists: can God be evil? The answer to this question was (broadly speaking) "yes", but Kabbalists have gone through many strange gyrations in an attempt to avoid what was for many an unacceptable conclusion. It was difficult to accept that famine, war, disease, prejudice, hate, death could be a part of a perfect being, and there had to be some way to account for evil which did not contaminate divine perfection. One approach was to sweep evil under the carpet, and in this case the carpet was Malkuth. Malkuth became the habitation for evil spirits.
If one examines the structure of the Tree without prejudice then it is difficult to avoid the conclusion that evil is quite adequately accounted for, and there is no need to shuffle evil to the periphery of the Tree like a cleaner without a dustpan. The emanation of any sephirah from Chokhmah downwards can manifest as good or evil depending on circumstances and the point of view of those affected by the energy involved. This appears to have been understood even at the time of the writing of the "Zohar", where the mercy of God is constantly contrasted with the severity of God, and the author makes it clear that one has to balance the other - you cannot have the mercy without the severity. On the other hand, the severity of God is persistently identified with the rigours of existence (form, finiteness, limitation), and while it is true that many of the things which have been identified with evil are a consequence of the finiteness of things, of being finite beings in a world of finite resources governed by natural laws with inflexible causality, it not correct to infer (as some have) that form itself is intrinsically evil.
The notion that form and matter are intrinsically evil, or in some way imperfect or not a part of God, may have reached Kabbalah from a number of sources. Scholem comments:
"The Kabbalah of the early thirteenth century was the offspring of a union between an older and essentially Gnostic tradition represented by the book "Bahir", and the comparatively modern element of Jewish Neo-Platonism."
There is the possibility that the Kabbalists of Provence (who wrote or edited the "Sepher Bahir") were influenced by the Cathars, a late form of Manicheanism. Whether the source was Gnosticism, Neo-Platonism, Manicheanism or some combination of all three, Kabbalah has imported a view of matter and form which distorts the view of things portrayed by the Tree of Life, and so Malkuth ends up as a kind of cosmic outer darkness, a bin for all the dirt, detritus, broken sephira and dirty hankies of the creation. Form is evil, the Mother of Form is female, women are definitely and indubitably evil, and Malkuth is the most female of the sephira, therefore Malkuth is most definitely evil...quod erat demonstrandum. By the time we reach the time of S.L. Mathers and the Golden Dawn there is a complete Tree of evil demonic Qlippoth underneath Malkuth as a relection of the "good" Tree above it. I believe this may have something to do with the fact that meditations on Malkuth can easily become meditations on Binah, and meditations on Binah have a habit of slipping into the Abyss, and once in the Abyss it is easy to trawl up enough junk to "discover" an averse Tree "underneath" Malkuth. This view of the Qlippoth, or Shells, as active, demonic evil has become pervasive, and the more energy people put into the demonic Tree, the less there is for the original. Abolish the Qlippoth as demonic forces, and the Tree of Life comes alive with its full power of good and evil. The following quotation from Bischoff  (speaking of the Sephiroth) provides a more rational view of the Qlippoth:
"Since their energy [of the sephiroth] shows three degrees of strength (highest, middle and lowest degree), their emanations group accordingly in sequence. We usually imagine the image of a descending staircase. The Kabbalist prefers to see this fact as a decreasing alienation of the central primeval energy. Consequently any less perfect emanation is to him the cover or shell (Qlippah) of the preceeding, and so the last (furthest) emanations being the so-called material things are the shell of the total and are therefore called (in the actual sense) Qlippoth."
This is my own view; the shell of something is the accretion of form which it accumulates as energy comes down the Lightning Flash. If the shell can be considered by itself then it is a dead husk of something which could be alive - it preserves all the structure but there is no energy in it to bring it alive. With this interpretation the Qlippoth are to be found everywhere: in relationships, at work, at play, in ritual, in society. Whenever something dies and people refuse to recognise that it is dead, and cling to the lifeless husk of whatever it was, then you get a Qlippah. For this reason one of the vices of Malkuth is Avarice, not only in the sense of trying to acquire material things, but also in the sense of being unwilling to let go of anything, even when it has become dead and worthless. The Qlippah of Malkuth is what you would get if the Sun went out: Stasis, life frozen into immobility.
The other vice of Malkuth is Inertia, in the sense of "active resistance to motion; sluggish; disinclined to move or act". It is visible in most people at one time or another, and tends to manifest when a task is new, necessary, but not particularly exciting, there is no excitement or "natural energy" to keep one fired up, and one has to keep on pushing right to the finish. For this reason the obligation of Malkuth is (has to be) self-discipline.
The virtue of Malkuth is Discrimination, the ability to perceive differences. The ability to perceive differences is a necessity for any living organism, whether a bacteria able to sense the gradient of a nutrient or a kid working out how much money to wheedle out of his parents. As Malkuth is the final realisation of form, it is the sphere where our ability to distinguish between differences is most pronounced. The capacity to discriminate is so fundamental to survival that it works overtime and finds boundaries and distinctions everywhere - "you" and "me", "yours" and "mine", distinctions of "property" and "value" and "territory" which are intellectual abstractions on one level (i.e. not real) and fiercely defended realities on another (i.e. very real indeed). I am not going to attempt a definition of real and unreal, but it is the case that much of what we think of as real is unreal, and much of what we think of as unreal is real, and we need the same discrimination which leads us into the mire to lead us out again. Some people think skin colour is a real measure of intelligence; some don't. Some people think gender is a real measure of ability; some don't. Some people judge on appearances; some don't. There is clearly a difference between a bottle of beer and a bottle of piss, but is the colour of the bottle important? What is important? What differences are real, what matters? How much energy do we devote to things which are "not real". Am I able to perceive how much I am being manipulated by a fixation on unreality? Are my goals in life "real", or will they look increasingly silly and immature as I grow older? For that matter, is Kabbalah "real"? Does it provide a useful model of reality, or is it the remnant of a world-view which should have been put to rest centuries ago? One of the primary exercises of an initiate into Malkuth is a thorough examination of the question "What is real?".
The Spiritual Experience of Malkuth is variously the Knowledge and Conversation of the Holy Guardian Angel (HGA), or the Vision of the HGA (depending on who you believe). I vote for the Vision of the HGA in Malkuth, and the Knowledge and Conversation in Tiphereth. What is the HGA? According to the Gnosticism of Valentinus each person has a guardian angel who accompanies that individual throught their life and reveals the gnosis; the angel is in a sense the divine Self. This belief is identical to what I was taught by the person who taught me Kabbalah, so some part of Gnosticism lives on. The current tradition concerning the HGA almost certainly entered the Western Esoteric Tradition as a consequence of S.L. Mather's translation  of "The Book of the Sacred Magic of Abramelin the Mage", which contains full details of a lengthy ritual to attain the Knowledge and Conversation of the HGA. This ritual has had an important influence on twentieth century magicians and it is often attempted and occasionally completed.
The powers of Malkuth are invoked by means of the names Adonai ha Aretz and Adonai Melekh, which mean "Lord of the World" and "The Lord who is King" respectively. The power is transmitted through the world of Creation by the archangel Sandalphon, who is sometimes referred to as "the Long Angel", because his feet are in Malkuth and his head in Kether, which gives him an opportunity to chat to Metatron, the Angel of the Presence. The angel order is the Ashim, or Ishim, sometimes translated as the "souls of fire", supposedly the souls of righteous men and women.
In concluding this section on Malkuth, it worth emphasising that I have chosen deliberately not to explore some major topics because there are sufficient threads for anyone with an interest to pick up and follow for themselves. The image of Malkuth as Mother Earth provides a link between Kabbalah and a numinous archetype with a deep significance for some. The image of Malkuth as physical substance provides a link into the sciences, and it is the case that at the limits of theoretical physics one's intuitions seem to be slipping and sliding on the same reality as in Kabbalah. The image of Malkuth as the sphere of the elements is the key to a large body of practical magical technique which varies from yoga-like concentration on the bodily elements, to nature-oriented work in the great outdoors. Lastly, just as the design of a building reveals much about its builders, so Malkuth can reveal a great deal about Kether - the bottom of the Tree and the top have much in common.