To expand on Deadbolt's mention of pass coverage, I'd like to discuss the coverages more in-depth

  • Zone: Typical assignments:
    • Cover Three: the most usual (that I've seen). The field is divided into thirds, for two cornerbacks and one free safety. The linebackers cover the flats.
    • Cover Two: slightly less well-known. The field is divided in half, with two safeteys (free and strong) and the two cornerbacks covering the flats.
    • Cover Four: unusual, also called the Umbrella coverage. You guessed it, field is divided into fourths, the corners on the outside and the safeteys on the inside. Can be turned into a Cover Two by sending either of the two back up to the flats.
  • Man-to-man: not much variations on this. Our defensive coach always said "You stay with him until that whistle blows. He goes to the bathroom, you wipe for him" and that's how close you have to stay with him, anticipating his moves.
  • Bump and run: is actually a kind of man-to-man, bumping him at the line then following him. Usually a second or two is all that it takes to end a receiver's chance of getting the ball, but if the bump doesn't buy you enough time, the coverage will discourage a pass.
and here are the zones:

          - - - - - - - - - - - -     (line of scrimmage)

              |               |
      C       |       S       |        C
              |               |

                 Cover Three

          - - - - - - - - - - - -     (line of scrimmage)

      C               |               C
              S              $

             Cover Two

          - - - - - - - - - - - -     (line of scrimmage)

          |              |             |
   C           S             $            C

              Cover Four