When non-Jews ask about Judaism, they commonly ask questions like: Do you believe in heaven and hell? In angels or the devil? What happens to the soul after death? What is the nature of G-d and the universe? The answers to questions like these define most religions; in fact, I have heard some people say that the purpose of religion is to answer these kinds of questions. Yet in Judaism, most of these cosmological issues are wide open to personal opinion. The areas of Jewish thought that most extensively discuss these issues, Kabbalah and Jewish mysticism, were traditionally not even taught to people until the age of 40, when they had completed their education in Torah and Talmud.
Mysticism and mystical experiences have been a part of Judaism since the earliest days. The Torah contains many stories of mystical experiences, from visitations by angels to prophetic dreams and visions. The Talmud considers the existence of the soul and when it becomes attached to the body. Jewish tradition tells that the souls of all Jews were in existence at the time of the Giving of the Torah and were present at the time and agreed to the Covenant. There are many stories of places similar to Christian heaven and purgatory, of wandering souls and reincarnation. The Talmud contains vague hints of a mystical school of thought that was taught only to the most advanced students and was not committed to writing. There are several references in ancient sources to ma'aseh bereishit (the work of creation) and ma'aseh merkavah (the work of the chariot [of Ezekiel's vision]), the two primary subjects of mystical thought at the time.
In the middle ages, many of these mystical teachings were committed to writing in books like the Zohar. Many of these writings were asserted to be secret ancient writings or compilations of secret ancient writings.
Like most subjects of Jewish belief, the area of mysticism is wide open to personal interpretation. Some traditional Jews take mysticism very seriously. Mysticism is an integral part of Chasidic Judaism, for example, and passages from kabbalistic sources are routinely included in traditional prayer books. Other traditional Jews take mysticism with a grain of salt. One prominent Orthodox Jew, when introducing a speaker on the subject of Jewish mysticism, said basically, "it's nonsense, but it's Jewish nonsense, and the study of anything Jewish, even nonsense, is worthwhile."
The mystical school of thought came to be known as Kabbalah, from the Hebrew root Qof-Beit-Lamed, meaning "to receive, to accept." The word is usually translated as "tradition." In Hebrew, the word does not have any of the dark, sinister, evil connotations that it has developed in English. For example, the English word "cabal" (a secret group of conspirators) is derived from the Hebrew word Kabbalah, but neither the Hebrew word nor the mystical doctrines have any evil implications to Jews.
Kabbalah is one of the most grossly misunderstood parts of Judaism. I have received several messages from non-Jews describing Kabbalah as "the dark side of Judaism," describing it as evil or black magic. On the other end of the spectrum, I receive many messages wanting to learn more about the trendy doctrine popularized by various Jewish and non-Jewish celebrities.
These misunderstandings stem largely from the fact that the teachings of Kabbalah have been so badly distorted by mystics and occultists. Kabbalah was popular among Christian intellectuals during the Renaissance and Enlightenment periods, who reinterpreted its doctrines to fit into their Christian dogma. In more recent times, many have wrenched kabbalistic symbolism out of context for use in tarot card readings and other forms of divination and magic that were never a part of the original Jewish teachings. Today, many well-known celebrities have popularized a new age pop-psychology distortion of kabbalah (I have heard it derisively referred to as "crap-balah"). It borrows the language of kabbalah and the forms of Jewish folk superstitions, but at its heart it has more in common with the writings of Deepak Chopra than with any authentic Jewish source.
I do not mean to suggest that magic is not a part of Kabbalah. There are certainly many traditional Jewish stories that involve the use of hidden knowledge to affect the world in ways that could be described as magic. The Talmud and other sources ascribe supernatural activities to many great rabbis. Some rabbis pronounced a name of G-d and ascended into heaven to consult with the G-d and the angels on issues of great public concern. One scholar is said to have created an artificial man by reciting various names of G-d. Much later stories tell of a rabbi who created a man out of clay (a golem) and brought it to life by putting in its mouth a piece of paper with a name of G-d on it. However, this area of Kabbalah (if indeed it is more than mere legend) is not something that is practiced by the average Jew, or even the average rabbi. There are a number of stories that discourage the pursuit of such knowledge and power as dangerous and irresponsible. If you see any books on the subject of "practical kabbalah," you can safely dismiss them as not authentic Jewish tradition because, as these stories demonstrate, this kind of knowledge was traditionally thought to be far too dangerous to be distributed blindly to the masses.
It is important to note that all of these magical effects were achieved through the power of G-d, generally by calling upon the name of G-d. These practices are no more "evil" than the miracles of the prophets, or the miracles that Christians ascribe to Jesus. In fact, according to some of my mystically-inclined friends, Jesus performed his miracles using kabbalistic techniques learned from the Essenes, a Jewish sect of that time that was involved in mysticism.
To give you an idea of the nature of Kabbalah, I will briefly discuss one of the better known, fundamental concepts of kabbalistic thought: the concept of G-d as Ein Sof, the Ten Sefirot, and the kabbalistic tree of life. This explanation is, at best, a gross oversimplification. I do not pretend to fully understand these ideas.
According to Kabbalah, the true essence of G-d is so transcendent that it cannot be described, except with reference to what it is not. This true essence of G-d is known as Ein Sof, which literally means "without end," which encompasses the idea of His lack of boundaries in both time and space. In this truest form, the Ein Sof is so transcendent that It cannot have any direct interaction with the universe. The Ein Sof interacts with the universe through ten emanations from this essence, known as the Ten Sefirot.
These Sefirot correspond to qualities of G-d. They consist of, in descending order, Keter (the crown), Chokhmah (wisdom), Binah (intuition, understanding), Chesed (mercy) or Gedulah (greatness), Gevurah (strength), Tiferet (glory), Netzach (victory), Hod (majesty), Yesod (foundation) and Malkut (sovereignty). The middle five qualities are mentioned explicitly and in order at I Chronicles 29:11: Yours, O L-rd, is the greatness (gedulah), the strength (gevurah), the glory (tiferet), the power (netzach), and the splendor (hod). I have seen this passage translated in widely varying ways, but the Hebrew corresponds to the names of the Sefirot in order.
The Ten Sefirot include both masculine and feminine qualities. Kabbalah pays a great deal of attention to the feminine aspects of G-d.
The Sefirot are commonly represented as in the diagram at left. This diagram is commonly known as the Tree of the Sefirot or the Kabbalistic Tree of Life. There is great significance to the position of these various attributes and their interconnectedness.
The Sefirot are not separate deities, as some might think by taking this too literally. They are intimately a part of G-d, and yet they are in contact with the universe in a way that the Ein Sof is not. The Sefirot connect with everything in the universe, including humanity. The good and evil that we do resonates through the Sefirot and affects the entire universe, up to and including G-d Himself.
Readings in this area should be undertaken with extreme caution. There is entirely too much literature out there under the name "Kabbalah" that has little or nothing to do with the true Jewish teachings on this subject. Any book on the subject of practical Kabbalah should be disregarded immediately; no legitimate source would ever make such teachings available to a faceless mass audience. Books written by Christians should be viewed with extreme skepticism, because many Christian sources have reinterpreted Kabbalah to fit into Christian dogma.
There is a nice online introductory Kabbalah course available from Aish.com at Kabbalah 101.
For an academic and scholarly information about Jewish mysticism and Kabbalah, check out the works of Professor Gershom Scholem. He was a prolific writer on the subject, and his writings are widely available and well-respected by both Jews and non-Jews.
For a more personal and experiential approach to Jewish mysticism and Kabbalah, see the works of Aryeh Kaplan. I am informed that his books are reliably authoritative and uncompromisingly Orthodox. I have found his materials on meditation and prayer, especially Jewish Meditation, to be particularly useful in my own devotional practices.
Of course, if you are serious about Kabbalah, you must get yourself a teacher that you can work with one-on-one, either online or in person.