Chassidism is a movement within Judaism founded by Rabbi Yisrael Ba'al Shem Tov (5458 – 5520 [1698-1760 CE]). Its purpose is to awaken the Jewish People to its own inner self through the study of the inner dimension of the Torah, which explores the inner dimensions of creation and Creator, thus preparing the way for the advent of Moshiach.

Chassidut is inwardly based upon the ancient doctrinal tradition of Kabbalah. Outwardly it gives new emphasis to the simple and joyful service of God, particularly through prayer and acts of loving-kindness. In Chassidic thought, the abstract and often impenetrable formulae of classical Kabbalah are recast into the psychological terms of human experience, making it understandable and accessible to the average person.

In Chassidic thought, the abstract and often impenetrable formulae of classical Kabbalah are recast into the psychological terms of human experience

Another way of explaining the differing emphases of Kabbalah and Chassidut is to say that Kabbalah focuses on the "vessels" (kelim) of Creation while Chassidut deals with the "lights"(orot) that fill these vessels. This distinction is apparent even in the names attached to these two mystical traditions: The word Kabbalah in Hebrew is derived from the root kabal, "to serve as a receptacle or vessel," while the word Chassidut is constructed from the root chesed, "lovingkindness," an attribute often referred to symbolically as the light of day.

The Ba'al Shem Tov brought Kabbalistic thought to its historical apex, both in terms of its conceptual refinement and its degree of influence upon the lives of the Jewish populace. It has been said that where Kabbalah is the "soul of the Torah," Chassidut is "the soul within the soul." The body of Torah (Talmud, Halachah etc) teaches you the what and how of Judaism, the soul of (and especially the soul of the soul of) Torah illuminates the why and what for.

See also If Chassidut is so important why wasn't it available until 300 years ago?