From Diablo Wiki
Demon Hunter's Traps
They function similar to the Assassin's traps in Diablo II where the Demon Hunter throws her device onto the ground and monsters in proximity will trigger its effect(s). These effects may range from simple damage to debuffs.
There is a bit of a twist to trap skills in Diablo III, however, as the Demon Hunter may only have so many of them out in the play field at any given time. It is currently unknown what the exact numbers will be, nor how many traps she will have in her kit. The only trap skill known at the moment is Spike Trap.