The following is a basic overview of the roles
Tanks
Jedi Knight - Guardian - Defense
Trooper - Vanguard - Shield Specialist
Jedi Counsular - Shadow - Kinetic Combat
Healers
Trooper - Commando - Combat Medic
Jedi Consular - Sage - Seer
Smuggler - Scoundrel - Sawbones
Melee/CQ DPS
Jedi Knight - Sentinel - Combat or Watchmen
Jedi Knight - Guardian - Vigilance
Trooper - Vanguard - Tactics
Jedi Consular - Shadow - Infiltration
Smuggler - Scoundrel - Scrapper
Range DPS
Trooper - Commando - Gunnery
Jedi Consular - Sage - Telekinetics
Smuggler - Gunslinger - Saboteur or Sharpshooter
*NOTE: Each classes Shared tree is not listed as they cannot be categorized
http://www.swtor.com/info/systems/advanced-classes
Legend
Class - Advanced Class - Advanced Class Specialization