Kabbalah is the mystical tradition of Judaism, and its characteristic instrument is a diagram of how an infinite God becomes a finite world.
The problem it addresses is genuine and old: if God is without limit, how does anything limited come to exist? The Kabbalistic answer is emanation through ten sephiroth, Keter the crown, Chokmah wisdom, Binah understanding, Chesed loving-kindness, Gevurah severity, Tiferet beauty, Netzach endurance, Hod majesty, Yesod foundation and Malkuth the kingdom, arranged in three columns and connected by twenty-two paths corresponding to the letters of the Hebrew alphabet. The unlimited source above them is Ein Sof, that which has no end.
The structure is read on several registers at once, and this is what gives it its reach. The sephiroth are stages of divine self-disclosure; they are also faculties of the human soul, since a human being is made in the image; they are also the anatomy of the created world. The same diagram therefore serves as cosmology, psychology and a map of ascent, and the practitioner works up it.
The Lurianic development of the sixteenth century, from Isaac Luria at Safed, added the elements most people now associate with Kabbalah: tzimtzum, the divine contraction that makes a space for creation; the shattering of the vessels, in which the emanated lights broke the containers meant to hold them and scattered sparks into the material world; and tikkun, the repair, in which human observance gathers the sparks back. That scheme gives ordinary religious action cosmic consequence, and it is why Lurianic Kabbalah spread so fast and so far.
Techniques include letter permutation, contemplation of the divine names, and gematria, the reading of numerical values of words.