He can either play up close to the line or back further. He helps on deep passes. Some teams call him a free safety. These players cover the wide receivers. They try to prevent passes being completed to receivers.
Teams usually use two cornerbacks, one on each side of the field.
A player that kicks off and kicks extra points and field goals. A player that kick the ball down filed to the other team on fourth down. Teams like to have a punter that kicks the ball high and far. This helps the defense. His job is to catch the ball and try to get as far as he can.