The Crucible is Destiny's competitive multiplayer game mode. In between story-driven, cooperative, or solo activities, Guardians can travel to orbit and select the Crucible, and immediately head into competitive play.
For investing time in the Crucible, players gain Crucible Marks (a currency) and Crucible Reputation, which in turn can be used to purchase special weapons and armor at the Tower. Players can also earn gear by participating in matches, but the specifics of what leads to these unlocks (playing, winning, accomplishing feats, etc.) is unknown.
Check out a Titan take on a team that obliterates the competition in this first gameplay video of Mars: Blind Watch in the Crucible.