Generally speaking, traps are quick-hitting runs meant to gain a few tough yards.
Traps compliment plays like the Zone Read and Power O to achieve a well-rounded ground game. And anyone on the offensive line can run a trap: one OL leaves his man intentionally unblocked, only to be picked up by a different lineman - often unseen by the defender.
The inside trap asks one guard (in the image below, LG A.Q. Shipley) to leave his man intentionally unblocked and move immediately to the second level. The other guard (below, RG Marshall Yanda) "trap" blocks, or comes around behind the center and takes on the defender vacated by the first guard.
The rest of the line tries to block out - away from the middle of the field - to give the running back as much space as possible.
Outside References
