What is a "pick-six"?

What is the position of the player who snaps the ball to the quarterback? Which defensive position lines up deep in the secondary as the last line of defense? 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 an onside kick? What is the name for a defensive play where extra rushers go after the quarterback? What is the term for a quarterback running the ball himself on a short-yardage push? What is a "flea flicker"?