Defensive back

In American football, defensive backs (DBs) are the players on the defensive team who take positions somewhat back from the line of scrimmage; they are distinguished from the defensive line players and linebackers, who take positions directly behind or close to the line of scrimmage.

The defensive backs, in turn, generally are classified into several different specialized positions:

The group of defensive backs is known collectively as the secondary. They most often defend the wide receiver corps; however, at times they may also line up against a tight end or a split out running back. They are usually the smallest, quickest players on the field.
 * Safety
 * Cornerback, which include
 * nickel back, the fifth defensive back in some sets
 * dime back, the sixth defensive back in some sets
 * The seventh defensive back, in the exceedingly rare 'quarter' set
 * known as a dollar back or a quarter back (not to be confused with the offensive player who throws the ball)