The kabbalah involves the study of a system called The Tree of Life or the ten Sephiroth. Each Sephirah involves a group of associated concepts. For instance, Geburah is associated with Isaac, "power," "judgment," the color red, the planet Mars, the concept of height, etc.; while Hesed is associated with Abraham, (fatherly) "love," "grace," the color white, the planet Jupiter, the element of fire, etc. The Tree of Life is an arrangement of the ten Sephiroth as the vertices of a graph. The 22 edges of this graph represent "paths" between the Sephiroth; these paths also participate in a series of correspondences, for example with the 22 Major Arcana of the tarot.
There are a variety of different interpretations of The Tree of Life as a metaphysical system. For example, the ten Sephiroth can be interpreted as stages of creation, from Kether, the divine singularity of God, to Malkhut the mundane work-a-day world of human experience. Instead of interpreting the Sephiroth as temporally distinct, however, we might alternately interpret them as materially distinct. Under this analysis, each Sephiroth represents a distinct "level of reality" or "plane of existence." Again, Malkhut corresponds to the work-a-day world of ordinary appearance (prison of the uninitiated), while Kether represents the transcendent level of the Godhead, where all is one.
A third, more deeply theological interpretation is also possible. Traditionally, the Sephiroth were taken to represent different aspects of the Divine. For example, Geburah would represent the judgmental, retributive aspect of God, while Hesed would represent his fatherly love.
As a metaphysical system, the kabbalah leaves little to recommend it. The original draw of kabbalistic thought for Western occultists was its supposed origin in antiquity. Until quite recently, the kabbalah was taken as an ancient Jewish system, dating back to the first and second centuries AD. The most important early kabbalistic work, however, the Zohar was written in the late 13th century AD, after Maimonides' attempt at Aristotelianizing the Jewish faith. The Zohar's mysticism can be seen more as an attempt to reinforce the distinctiveness of Jewish culture in the face of widespread persecution and assimilation than as a legitimate recovery of ancient practices. This, at least, is the view popularized by Gershom Scholem.