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Kabbalah is the main and most widespread form of Jewish mysticism and esoterism. Despite being a deep and complex aspect of Orthodox Judaism, it has been taken up by several celebrities, reducing an otherwise valid concept (for a religion) to wearing a red string around your wrist.
Kabbalah has its own system of ritualistic sex, similar to Indian tantra. It is known as Karezza, and is apparently encoded in much of western art and literature  Kabbalah, unlike traditional doctrine of Orthodox Judaism, teaches a belief in reincarnation. Adam was created by a hermaphrodite, like God/Goddess. 
Not real Kabbalah
Seeing a mystical belief system that hadn't been bastardised by any woo-peddlers yet, the folks at the Kabbalah Center took it upon themselves to create just that: a bastardized, inaccurate rip off of Kabbalah. Notable "practitioners" of this celebrity-friendly religion include Madonna and her ex-husband Guy Richie who overused Kabbalic ideas in his film Revolver.
Some Christians adopted elements of Kabbalah from Judaism, from the 13th centuries onwards. In many cases, this was motivated out of a belief that Kabbalah could be used as a tool to convert Jews to Christianity. The Latinised spelling Cabala is sometimes used to refer to this Christian variant. Over time Christian Cabala blended with other strands of Western esotericism/hermeticism; to refer to its hermetic forms (especially that taught by the Hermetic Order of the Golden Dawn), the spelling Qabalah is used (which in itself is an alternative romanisation of the Hebrew).
Claims from Marcus Weston that the British Royal Family practised Kabbalah have made their way onto the neo-Nazi sphere of the internet, as proof of the international Jewish conspiracy, of course!
Kabbalic references are also frequently used in Japanese media for the sole purpose of looking cool (see Neon Genesis Evangelion and Fullmetal Alchemist).
Actual Kabbalistic beliefs are complex and nearly impenetrable by most ordinary people, seeming to be quite different from mainstream Judaism, and has a bit more in common with Dharmic religions than other Abrahamic religions.
One of the main differences from mainstream Judaism is that they Kabbalah includes a kind of reincarnation, which posits that the souls of every Jew that has ever lived are the souls of the Israelites that were present when they accepted the Covenant with God. Non-Jews have, well, non-Jewish souls. Though the existence of converts to Judaism seems to present a problem with this belief, Kabbalists offer the explanation that they are merely Jewish souls "trapped" within a non-Jewish body. The Jewish souls are said to go through this cycle of reincarnation for an unspecified amount of time until they live a life in which they fulfill all 613 mitzvot (commandments), when they then ascend to the HaOlam HaBa (Literally, "The World To Come").
The Big Guy
Kabbalists usually believe that God is neither a man nor a woman, and therefore refer to him using the gender-neutral "It" rather than the masculine "He." They, much like Muslims, take the whole "not-ascribing-human-traits-to-God" thing more seriously than most, and hold, much like Islam, that the nature of God is completely unknowable and indescribable. They hold that the "God" that most Jews worship isn't really God himself, but instead the ten sefirot, or the ten characteristics of God. The Real God™ is what they call the Ein Sof ("The Endless One"), and the sefirot are merely the qualities that God manifests Himself in reality through. According to Kabbalists, this is most definitely NOT any kind of belief in the duality of God.
- Kaufmann Kohler, Louis Ginzberg; Kabbalah, JewishEncyclopedia.com (1906)
- Joseph Telushkin, Kabbalah: Table of Contents, Jewish Virtual Library
- Rabbi Tzvi Freeman, What Is Kabbalah? - The Soul of Judaism, Chabad.org
- Kabbalah Online, Chabad.org
- A rabbi discusses why celebrities like Kabbalah
- Anyone who sufficiently cares can work their way through this article.
- Which is also used to collect donation from tourists visiting Israel...
- The secret history of the world: As Laid Down By The Secret Societies, Mark Booth, 2008, Page 12
- Kabbalah: The Mystic Quest in Judaism, David S. Arie,, Page 137
- Wikipedia on Revolver's Kabbalic references.
- Oliver, Charlotte. "Royal family members follow Kabbalah" Jerusalem Chronicle. Published May 19, 2015. Screenshot available here.
- "Members of British royal family are kabbalists", via a blog so delightfully called Diversity Macht Frei.