Characters are divided into ten individual classes, each of which has their own playstyle and abilities. What class you play revolves very much upon how you like to play.
Former servants of the Lich King, Death Knights are able to either Tank as Blood, or DPS as either Blood, Frost or Unholy. They specialize in inflicting diseases to their opponents, and punishing them with heavy-hitting abilities.
Druids commune with nature and natural world, and are able to adopt a myriad of forms to suit a chosen role. Druids can Tank as Feral, DPS as either Balance or Feral, and can also Heal as Restoration.
Priests use the power of the Holy Light to repair the wounds of their allies, while also tapping into the power of the shadows to vanquish their enemies. They can DPS as Shadow, and Heal as either Discipline or Holy.
Rogues are the only class that can open locked boxes, with their Lockpicking skill.
Shamans commune with the elements, and specialize in using Totems to assist themselves, or fellow party and raid members. They are capable of DPS as either Enhancement or Elemental, and can Heal as Restoration.
Warlocks tap into the Fel shadow magic of the nether planes, unleashing the very power of the shadows themselves. Warlocks can also summon a Demon under their command. They are a pure DPS class, capable of damage as Affliction, Demonology, or Destruction.