As in any MMO, the first thing you do is create your own avatar.
To begin this process in SW:TOR, you have the choice of one of two factions: The Republic (good guys) and the Empire (bad guys). Apologies to the Star Wars fans, who would obviously know this, but there are actually people out there who are unfamiliar with Star Wars lore.
Should you decide to play on the side of the Republic, you have a choice of the following classes: Smuggler, Jedi Knight, Commando or Jedi Counselor.
Playing on the seductive Dark Side of the Force (Empire), you may choose between: Imperial Agent, Sith Warrior, Sith Inquisitor and Bounty Hunter.
Now while it may seem that you have the classic tank, damage dealer and healing classes; you don’t. The twist is that, as soon as a character reaches level 10, there is an additional choice between two specializations. For example; a Sith Warrior can decide to play either as a Juggernaut (a strong warrior that specializes in up-front combat) or as a Marauder (a warrior that dual-wields light sabers and can destroy bulks of enemies with the sleight of his hand, but cannot take as much damage as the Juggernaut).