A blitz is a play in which the defending team wants to apply pressure to the opposing team's quarterback. A corner blitz is when one of the cornerbacks come from outside to bring this pressure to the quarterback hoping to induce a poor pass, interception, sack, or incompletion. This type of play is usually a high risk, high reward situation as a cornerback leaving the pass defense leaves holes behind him.
A corner blitz is different from the other types of blitzes because the corner usually comes from the blindside of the quarterback, bringing an element of surprise. The surprising pressure is supposed to be a change of pace that allows the defense to reach the quarterback for a sack or cause him to make a poor decision throwing the ball. A cornerback is also a generally faster player than a linebacker or defensive end so by combining speed and surprise he should have a good chance to create a positive play for the defense.
In order to blitz the quarterback, a corner leaves his assigned receiver which causes a shift in the defense. If this shift in the defensive coverage is not done properly and the blitzing corner is not successful in reaching the quarterback in time, the play will usually result in a large gain for the offense. To counter this, defenses generally use corner blitzes on as a change of pace rather than a regular defensive play.