A defensive back in football is a defensive player who lines up in the defensive backfield with the primary objective of defending against the pass. Defensive backs include cornerbacks and safeties. Linebackers may also be considered defensive backs at times.
Defensive backs defend against offensive players whose primary goal is to catch passes, such as wide receivers and tight ends. Cornerbacks usually play against wide receivers while safeties play against tight ends and running backs, as well as playing deep zone defense to prevent long passes.
Defensive backs must be very quick and agile to defend against the pass, but be quick enough mentally to distinguish between passing plays and running plays.