"Over 250 years ago, a revolutionary movement arose at the center of the Jewish world. With remarkable speed, the Hasidic movement spread throughout Eastern Europe and White Russia (now Belarus). Hasidism had a profound - and continuing - impact on Jewish spiritual thought and practice, changing it as no other movement has.
What is Hasidism? What is its innovation? Hasidism strives for consciousness of one's inner essence and simplicity - in relation to Torah, man, and divinity - and for this, there are no adequate words or direct definitions. Initially, Hasidism was an all-encompassing approach to life, a distinct way of praying, studying, and living that emphasized cleaving to and serving God with joy. Because it deals with man's inner essence, Hasidism defies easy definition or description. Our understanding is further complicated by the fact that the first generations of Hasidic masters, on principle, wrote little or not at all.
Even the few writings we do have tend to be secondary sources, often fragmented and unsystematic, and are idiosyncratic to the specific environment in which they arose. Thus, they provide faint illumination, while essentially leaving us in the dark."
Excerpt from Preface to Opening the Tanya by Rabbi Adin Even Israel Steinsaltz