According to msn.foxsports.com:

” One-Gap vs. Two-Gap

As important as the techniques are, gap responsibilities are much more vital to the success of the defense. Depending on the play called and the philosophy of the defensive coordinator, a defensive lineman could be responsible for either one or two gaps.

One-gap responsibility is relatively simple: the defender attacks a hole and must take care of whatever business happens there. He is expected to tackle any running back who goes through that hole, or to force the running back to move laterally into the arms of another tackler. If the offense is passing, the defender’s gap is his route to the quarterback.

Two-gap responsibility requires more discipline on the part of the defender. A defensive tackle may be responsible for both the A and B gaps on his side of the field. His job is not so much to crash through a gap as to read the play, anticipate which gap a running back might choose, and clog it. The two-gap defender must quickly diagnose the blocking scheme to determine which of his gaps is more vulnerable. ”