"The mystical form of Judaism is Kabbalah. Broadly speaking, Kabbalah refers to Jewish mysticism dating back to the time of the second Temple. For many years a carefully guarded oral tradition, it became systematized and dispersed in the Middle Ages. The kabbalistic viewpoint was expressed most importantly in the Yalkut Re'uveni by Reuben Hoeshke in 1660, but also made its way into prayer books, popular customs and ethics. The focus of the Kabbalah is the simultaneous transcendence and immanence of God, with the latter described in terms of the sefirot, or attributes of God."
From: Religionfacts.com website