Cornerback

A Cornerback (CB) (also called: Corner, and also referred to as Defensive Back) is a Defensive Position, played in the backfield, 5-10 yards behind the line of scrimmage, near the sidelines.

Responsibility
The responsibility of the Corner is to make an Interception on any passing plays (or at least block the passes) from the Wide Receiver, or if it's a running play, his responsibility is to tackle the Running Back in the offensive side.

Blocking a Pass
When the Quarterback passes the ball, there will always be a short time when the ball is in the air. You should always try to make an Interception no matter what happens. If you know that you CANNOT make an Interception, then you can always hit the ball out of the way, before reaching the intended Wide Receiver. This will make it the next down.

Blocking a Runner
When the Quarterback pitches the Football to anyone running (Generally the Running Back), the runner will run to the side lines, to prevent any tackles on him. This makes it where half the team, will not be able to catch him, as he has an advantage of speed. As the Wide Receiver tries to block you, the Running Back is running beside you. If he gets in range of you (which he most likely will, while you get closer to him) you can perform a good tackle. If he passes you, you will have to stop messing with the Wide Receiver, and chase him down. But you are NOT alone. The Safety which is behind you about 5-10 yards, will also have a chance to tackle him. If he at least slows him down, and you are chasing him, you will be able to perform a tackle on him.