When a fumble occurs on the field, or the ball hits a player unexpectedly in certain situations, the ball is live and any team may claim it by picking it up. When a player attempts to claim possession of a loose ball and misses, it is called a muff.
In the case of a fumble, either team can claim the ball and a muff occurs when a player attempts to recover it and misses. This happens often as the oblong shape of the football causes many odd bounces, and the 100% leather surface can make the ball hard to grasp securely. A similar situation is the case of a muffed punt. A muffed punt occurs when a player on the kicking team interferes with the return team's ability to catch the ball, or is mistakenly hit by the ball. In those two cases, a muffed punt has occurred but the recovering team may not attempt to return it after recovery. A muff is different than a fumble because a player must possess the ball in order to fumble it, one cannot fumble a loose ball, which is why it is referred to as a muff.