Have been beaten to death by the FAQs.
But simply:
Marauder: Melee DPS only, several different specs but all melee and like 2 force powers total. Dual wield two single hilt sabers.
Juggernaut: Melee DPS, can tank IF SPECS tank. All melee, like 2 force powers. 1 single hilt saber.
Assassin: Melee DPS, can tank IF SPECS tank. Has some force powers, cool animations, but their range is basically 10m or less so its all basically melee range with cool graphics. Uses a saber-staff.