It is important to differentiate between the popular notion of Kabbalah and the concept within traditional Judaism. In the popular culture, Kabbalah is perceived as a form of magic or the occult, studied for selfish personal gain. This misinformed idea resulted from those who adapted Jewish ideas out of the context of Jewish belief and practice, warping it away from its foundations to their own purposes. These include medieval Christian mystics, neo-pagan groups, and contemporary "new age" movements.
Within Judaism, though, Kabbalah is the part of Torah that addresses the process of creation ("Ma'aseh B'raisheet") and the relationship that God maintains with creation ("Ma'aseh Merkavah"). As such it is the Torah's inner aspect. Some traditions say that some of the key texts go as far back as the Patriarch Abraham.
Parts of Kabbalah, such as the Zohar and Rabbi Moshe Cordovero's "Pardes Rimonim," are accessible but difficult to understand without a firm grounding in the more basic Jewish sources and an informed teacher. Other parts remain hidden and unavailable to the public. Parts have been committed to print but others remain as closely held, orally transmitted tradition.