From Chabads 'Ask Moses' site @
http://www.askmoses.com/en/article/244,203/What-is-Chassidism.html#articlepage
What is Chassidism?A. Chassidism is the Judaism-revolutionizing, Moshiach-bringing movement that swept across and overtook classical Judaism in the early-to-mid 1700s. It was founded by Rabbi Israel Baal Shem Tov.
B. Chassidism is to Judaism what a second kidney is to the man who only has one, or what color is to black-and-white photography—an enhancement. An improvement. A dramatic betterment of the status quo.
C. Chassidism came to Jewry when Jewry couldn’t have possibly needed it more. Like a doctor whispering a fainted man’s name in his ears, the Nation of Israel, devastated by the Cossacks and the false messianism of Shabbatai Tzvi, received a wake-up call when G-d whispered “Israel Baal Shem Tov” into its collective ear. During his lifetime, the Baal Shem Tov recorded an out-of-body experience in which he returned to Heaven to ask Moshiach when he would come. Moshiach responded: “When your wellsprings will spread out,” meaning: when your teachings change the Jewish world. Chassidism is thus a cataclysmic, cosmic development, laying the groundwork for the arrival of Moshiach itself.
What defines Chassidism?
1. The Movement
Chassidism began in 1698 with a soul named Israel descending from Heaven into the body of the yet-to-be-born son of Eliezer and Sarah, a kindly couple of the little village of Okup, Ukraine. Little Israel’s father died young, charging his only child to “fear nothing but G-d.” His mother died a short while later.
Having little formal education, the lad joined a small school as a teacher’s apprentice, spending much free time wandering the fields and forests pondering G-d’s greatness. As a teenager, Israel joined a wandering group of mystics, becoming well versed in Kabbalistic traditions of Torah and acquiring the title “Baal Shem Tov,” or “Master of the Good Name.”1
At age 36, after years of traipsing about, the Baal Shem Tov was informed from On High that the time had come to change the world. He began addressing the illiterate peasant masses among the greater Jewish community, attracting a huge following among the learned and laymen alike with his inspirational tales, warmth, love and miracles. He taught that every Jew is beloved to G-d like an only son to his parents, and that innocence born of ignorance counts. He sweetened Judaism, counteracting the severity and elitism that the scholars and preachers had stratified the Jewish community with.
Eventually settling in Mezhibuzh, the Baal Shem gathered about him a hardcore inner circle of devoted students, all world-class Torah scholars in their own right. Upon his passing on the holiday of Shavuot, 1760, Rabbi Dov Ber, his finest disciple, a.k.a. the Maggid (Preacher) of Mezritch, assumed leadership of Chassidism. Under the Maggid, the third generation of Chassidic leadership, numbering several dozen brilliant students, studied, learned and developed individual interpretations of The Chassidic Idea. Upon the Maggid’s passing, leadership of the movement split evenly among these several dozen students, ultimately branching into several hundred schools of thought by the mid-1900s.
2. The Chassid
When Chassidism was born, it wasn’t called Chassidism. The title came later. What it means and what it came to mean are two different things. What it means is “piety.” What is means today is the same thing it meant when the early adherents of Chassidism adopted it for themselves: a spiritual, emotional devotion to the study of Torah and performance of mitzvahs above and beyond the letter of law. To the Chassidim, it wasn’t what you did as much as how you did it—with true happiness, positivity and good cheer. Today, a “Chassid” is a Jew who is born into a Chassidic family of any of the hundreds of existing Chassidic groups, and/or who follows or subscribes to that lifestyle.
3. The Rebbe
The Rebbe (pronounced REH-beh) is the spiritual axle that makes the wheel of Chassidism go ‘round. The first Rebbe was the Baal Shem Tov himself. The second was the Mezritcher Maggid. And the rest is history. Every Chassidic group has its own Rebbe, who becomes rebbe by being a member of the “royal dynasty” that the previous Rebbe belonged to. For example, the sixth Lubavitcher Rebbe, Rabbi Joseph I. Schneersohn, was the son of the fifth. His successor, the seventh Lubavitcher Rebbe, was his son-in-law and distant blood relative.
A Rebbe is a spiritual mentor. He counsels his followers in the ways of Chassidism in general, and in the ways of the group he helms in particular. If you’ve watched the Star Wars movies and are familiar with the Jedi Master concept, you have a slight inkling of what a Rebbe is all about. A rebbe is seen as the most perfect example of living the Jewish lifestyle according to Chassidic ideas and ideals. He is a role model. As such, he is also considered a tzadik, an innocent, highly spiritual visionary with the ability to effect miracles and perceive the extrasensory.
4. The Methods
The essential difference between Chassidism and all other streams of Orthodox Judaism is that while the other streams see Avodah (service of G-d) as part of Torah study, Chassidic Judaism sees Torah study as part of Avodah.
While many Jews places primary emphasis on Torah study, Chassidic Judaism places primary emphasis on Avodah. And within Avodah, several key schools of thought emerge. The Breslover Chassidim, for example place a heavy emphasis on raw communion with G-d—just going out into the woods for long stretches of thought. The Karliner Chassidim believe in high-decibel tefillah. The Satmars stress Torah study and kindness. Gerrer Chassidim stress the importance of truth, and, among many groups, try to stand as physically close to their Rebbe during services, the better to emulate him. And Lubavitcher Chassidim emphasize a disciplined intellectual/meditative discovery of G-d.
Virtually all Chassidic groups, from the Baal Shem himself on down, believe in using the pleasures of song and dance for spiritual gain. Every Chassidic dynasty thus has its own distinct repertoire of niggunim (wordless melodies, pronounced nih-GOO-nim), and incorporate much singing and dancing in their service of G-d.
Above all, the ideas of each Chassidic group, its particular flavor of Chassidism, is captured in print by the scholarly writings of its Rebbes. These books are studied throughout the Chassid’s life, as he constantly strives to make himself a better person.