Chassidism was basically the reform movement of the 19th century. There were many sects as noted and the local rav always has a special place as the representative of that authority, but centrality around the rabbinic dynasty is actually the smaller part of the equation than the newly inclusive, intuitive, spiritualist, and distinctly joyful impulse that drove the initial branching centering around an intensely affable mystic, the Baal Shem Tov.
There's a wonderful compendium of tales by Elie Wiesel called Souls on Fire on the subject first of that, and all the fantastically intense people and personalities that drove it's evolution into yet more branches.
The externally visible aspects are small in modern terms, but the size of the philosophical split is hard to overstate. And the chassids you're most likely to run into, Chabad-Lubavitch that are hardly the fearful hermits people are thinking of here. They value identity, but teach with a positivist tone and go out to and bring non-observant Jews to observance all the time and everywhere.