There are a total of 4 playable classes on the Sith / Empire side of Star Wars: The Old Republic at launch, though being an MMORPG (Massively-Multiplayer Online Roleplaying Game) there is a liklihood or more being added in the future.
The Classes are:
(Click Images for class page)
Bounty Hunter
Sith Warrior
Imperial Agent
Sith Inquisitor