ONSIDE KICK

An onside kick is an attempt by the kicking team to regain possession of the ball after a normal kickoff, rather than allowing their opponent possession of the ball.
An onside kick differs from a normal kickoff in two ways: 
First, the onside kick is usually kicked in a certain direction, usually diagonally or with an end over end spin close to the receiving team.
Second the onside kick has it’s own formation.  All ten players on the kicking side first line up in the standard kickoff formation.  Then, prior to the kick, most of the players shift to one side of the kicker or the other. This shift increases the odds that someone from the kicking team will be able to regain possession of the ball after it is kicked.

In the National Football League, an onside kick must travel 10 yards down field before the kicking team can touch it unless the receiving team touches it first within 10 yards.  If a member of the kicking team attempts to recover the ball before it has traveled 10 yards, and it has not been touched by a member of the opposing team, the recovery does not count.  If an attempt at an onside kick goes out of bounds untouched, it results in a five-yard penalty.
An onside kick cannot be advanced by the kicking team unless a member of the receiving team had possession of the football and lost it by turning the ball over.  A kick that is recovered after going 10 yards untouched cannot be advanced.

The best way to identify an onside kick is to watch for a shift in formation of the kicking team prior to the ball being kicked and the ball is deliberately kicked at an angle or close to the receiving team.