Skewers are closely related to pins. In fact, they look very similar, but work in reverse. In a skewer, a valuable piece is attacked and forced to move to avoid being captured, leaving a less valuable piece behind it free for capture. Like pins, skewers can only be created by the long-range pieces.
In the diagram above, the Black king is being checked by the White rook. Black's king can easily move to safety, but doing so allows White to play Rxe8, winning the queen.
