Titans are the 'tanks' of the three classes. Most of their abilities happen to deal with taking hefty loads of damage and dishing out plenty of damage in return. Titans are the only class to be able to shield bash another guardian to death in a single hit, along with surpressing abilities of other guardians and enemies.
Warlocks are the mages, or wizards, of the three classes. All of their abilities heavily deal in magic, such as a Stormcaller Warlock, or a Void Walker Warlock. Warlocks melee abilities are the longest reaching of the three classes. They're able to summon rifts that heal or empower their allies continuously for a short period of time.
Hunters are the rogues of the three classes. All hunter abilities deal with accuracy or speed, such as a Gunslinger Hunter or an Arcstrider Hunter. Hunter's abilities mainly deal with different weaponry, such as the fan of knives or the tempest strike.