In chess, a discovered attack is an attack revealed when one piece moves out of the way of another. The discovered check is a subset of this, where the unmasked attack is a check (conceivably, the piece moving away can also give check; in that case a double check results). Discovered attacks in general, and discovered checks in particular, can be extremely powerful, as the piece moved can make a threat independently of the piece it reveals. Like all chess tactics, they succeed because the opponent is unable to meet two threats at once.
When the piece unmasking the attack gives check to the opponent's king, the maneuver is often described as a discovered attack with check.
Sometimes discovered attacks - and especially discovered checks - can win material when the piece doing the "discovering" outrightly captures an opposing piece which is nominally protected by another opposing piece; since the opponent is preouccupied with getting the king out of check, the attacking player will have time to move the discovering piece out of harm's way. This scenario is often referred to as a discovered check (or attack) with capture.
Still another possibility is that the unmasking piece, rather than attacking or capturing an opposing piece, moves to a square from which it threatens to inflict checkmate on the next move, producing a discovered attack with mate threat.
Discovered attacks don't have to gain material to be effective; sometimes the tactic is used merely to gain a tempo.
The diagram at left shows a possible line in the Advance Variation of the French Defense in which Black has mistakenly attempted to win White's d4 pawn. Now White plays 1.Bb5+, a discovered attack on the queen. Whatever Black does, White will follow up with 2.Qxd4.
Note that this is technically not a discovered check, because the check was delivered by the moving piece, not the unmasked piece, but the effect is just as devastating.
See also algebraic chess notation