Yay! Prestige classes!
Info:
Jedi Weapon Masters are considered the best fighters in the galaxy. They can take a lot of damage before going down, and rarely surrender. They're the first choice for a field commander, and sometimes carry two lightsabers or a lightstaff. Weapon Masters usually start off as Guardians.
Jedi Watchmen are Jedi who watch over a particular part of the galaxy. I.E., Yoda was Watchman of Kashyyyk. Obi-Wan wasn't a formal Watchman, but he did oversee Luke on Tatooine. They're more in-tune with the Force than Weapon Masters. Watchmen typically make their beginnings as Sentinels.
Jedi Masters are just what they sound like. They like to work things out verbally rather than with "aggressive negotiations". They like to use the Force more than anything else, defensively, of course. Masters are generally Consulars at first.
Sith Marauders are equivalent to Weapon Masters in terms of fighting, but posses one more attribute: Rage Mode. Once unleashed, the Marauder goes crazy and kills everything in his/her path. Basically, they're all a bunch of team killers.
Sith Assassins are... assassins. They use their balance of Force and strength to kill opponents quickly and cleanly. They don't normally last long in an all-out battle, but watch your back.
Sith Lords are usually the heads of an army, owning to their instinctive ruthlessness and wisdom. They do what they have to to win, nothing more and nothing less. Like Jedi Masters, they like the Force, but use it far more aggressively. Think Darth Sidious.
For me, Weapon Master. They are invincible.