A fumbled ball may be recovered and advanced by either team (except, in American football, after the two-minute warning in either half or 4th down, when the fumbling player is the only offensive player allowed to advance the ball, otherwise the ball is ruled dead at the spot of recovery if the ball bounces backwards or spotted at the point of the fumble if the ball travels forward). It is one of two events considered to be turnovers, where possession of the ball can change during play.
Under American rules a fumble may be confused with a muff. A muff occurs where a player drops a ball that he does not have possession of, such as while attempting to catch a lateral pass or improperly fielding a kicking play such as a punt (you can't "fumble" a loose ball). The result is the same and most announcers will still call it a fumble. Ball security is a term used to describe the ability of a player to maintain control over the football during play and thus avoid a fumble