Cornerbacks are defensive players. They cover receivers, and defend against pass offenses and make tackles. The cornerback is expected to defend against the forward pass, anticipate the quarterback, and disrupt pass routes and execute man-to-man and zone coverage.
Zone coverage requires a cornerback to defend an assigned area of the field. Man-to-man coverage means the cornerback is responsible for covering a particular receiver who has been assigned to him.
Cornerbacks can be anywhere on the defensive side of the line scrimmage at the start of play, so there are no “illegal defense” formations.
A cornerback who is good at his position and can shut down an entire side of the field is called a “shutdown corner.”
Now that you know more about the position, you can better understand the jobs of players like Darrelle Revis and Nnamdi Asomugha, who are considered the top two cornerbacks in the NFL currently.