Dota and Dota 2 (indeed all MOBA games), rely on gameplay consisting of a myriad of heroes, each who fill one or more roles on the battlefield. These roles are primarily defined by their core attributes, various skills, and item builds. Some heroes naturally lean towards one or more roles, many of which are fluid and serve to offer some guidance regarding itemization, in-game behavior, and potential play styles.
Roles include: Edit
Carry: Excels in late-game teamfights, "carrying" the team forward to the win.
Disabler: Capable of stunning or otherwise inhibiting enemy players from casting abilities or moving.
Ganker: Assassins, capable of severely limiting enemy movement while unleashing heavy early-game damage.
Initiator: Capable of initiating team fights with large area-of-effect abilities, the ability to survive heavy barrage, or both.
Jungler: Effectively farms neutral creeps in early-game, allowing one hero to consume a lane's experience and gold alone.
Pusher: Excels at tearing through creep waves in lane and destroying enemy towers quickly.
Roamer: High mobility hero who can establish map dominance by harassing enemy heroes and warding the map to increase player visibility.
Support: Capable of supporting team mates with buffs, disables, heals, and the like. Ideal support players are not item dependent and therefore can allow the team carry to acquire a disproportionately large share of creep gold.
Tank: Capable of withstanding heavy enemy fire with large amounts of health and armor. These heroes seldom put out much damage up front, but can take sustain damage and draw enemy attention on themselves and away from the friendly carry. They usually become a threat over a period of time, dealing consistent damage.