[1] A classic reverse typically begins as a bootleg, sweep or end-around, but before the ball-carrier crosses the line of scrimmage he hands the ball off to a teammate, usually a wide receiver, running in the reverse (opposite) direction.
Because many of the defensive players will have gravitated in the direction of the original rusher, if the second ball-carrier can outrun the defenders to the other side of the field, he has a very good chance to make a big gain.
A variation of this play is a double reverse, in which the second ball-carrier takes the ball all or part way back across the field before he too hands off to a teammate running in the opposite direction.
A double reverse adds another level of surprise to the defense; however, the play takes more time and space to develop and increases the risk of a big loss or a fumbled handoff.
Sometimes an end-around, in which the only handoff is from a stationary quarterback to a wide receiver, is imprecisely called a "reverse".