Which defensive position lines up deep in the secondary as the last line of defense?

How many teams are in the NFL? What are the two conferences of the NFL called? How many divisions does each NFL conference have? What is the position of the player who snaps the ball to the quarterback? What is it called when a defender tackles the quarterback behind the line of scrimmage? What is it called when a defender catches a pass thrown by the offense? What is a "pick-six"? What is an onside kick?