A defensive or offensive position on the line of scrimmage.
On offense, the player snapping the ball is the center. The players on either side of him are the guards, and the players to the outside of him are the tackles. The players on the end of the line are the ends. This may be varied in an unbalanced line.
On defense, the outside linemen are ends, and those inside are tackles. If there are 5 or 6 linemen, the inner most linemen are known as guards. This is rare in professional football except for goal-line defense, but is sometimes seen in high school or college.