In chess, a skewer (or thrust) is an attack upon two pieces in a line and is similar to a pin. In fact, a skewer is sometimes described as a "reverse pin"; the difference is that in a skewer, the more valuable piece is in front of the piece of lesser value. The opponent is compelled to move the more valuable piece to avoid its capture, thereby exposing the less valuable piece which can then be captured. Queens, rooks, and bishops can skewer.
In the diagram to the left, with Black to move, the black queen is skewered by White's bishop. Black must move the queen, and on the next move, White will capture the rook. This is a relative skewer; Black is unlikely not to move the queen, which is more valuable than the rook--but the choice is still available.
In the diagram to the right, with White to move, the white king is skewered by the black bishop. This is an absolute skewer, because the rules of chess compel White to move out of check. After White chooses one of the handful of legal moves available, Black will capture the white queen.
Because the skewer is a direct attack upon the more valuable piece, it is generally a much more powerful and effective tactic than the pin. The victim of a skewer often cannot avoid losing material (though it may be possible if, for example, the more valuable piece can be moved with check); the only question is which material will be lost. The skewer occurs less often than the pin in actual play. When it does occur, however, it is often decisive.