A skewer is similar to a pin but with the logic reversed. You attack two pieces in a line, and often one of them is the enemy king; but instead of the king being behind a piece that is paralyzed in front of it, the king is in front of one of its fellow pieces, and is forced by an attack to step out of the way and allow the piece behind it to be captured. Or instead of the king it may be a queen or other piece that starts in front and must yield to the attacker. The point is that since your opponent is required to spend a move relocating the piece in front, you get a free shot at whatever was behind it. (In the diagram, Black can administer a skewer with Bg7. We'll return to the example in a minute.)
Since a skewer is based on the idea that a valuable piece in front is forced to jump out of the way of a less valuable piece behind it, we can make some useful generalizations about where we might find one. A skewer almost always will run through a king, queen, or rook (just as a pin almost always has a king, queen, or rook at the back end of it); and like pins, skewers can be inflicted by the bishop, rook, or queen. The instinct for spotting skewers also is similar to the instinct for seeing pins: become sensitive to any enemy pieces (or any pieces of your own) arranged on a line; trace the paths between pieces in search of opportunities.
The simplest skewers to analyze run through the enemy king, so we'll start by looking at some positions based on that model. Skewers of this sort tend not to occur until the middle or later parts of the game. The reason is that for the pattern to arise, either the enemy king has to get out in front of the target piece or the skewering piece has to get behind the king; one way or another, in other words, the king has to be between one of your pieces and one of his, and this generally isn't possible early in a game. Occasionally such a skewer can be administered along the back rank, as we shall see, but usually only after many of the king’s fellow pieces are off the board. Skewers through the king thus are an especially potent weapon in an endgame, when the king may be put on the run and your pieces have more ways to get behind it.
There are two general ways to find skewers, just as with any other tactic: by spotting the visual pattern or by asking questions and finding a skewer in the answers. The key visual pattern in this first example is the presence of White’s king and queen on the same diagonal; this always should raise a red flag. But if you don't see a pattern like this, you still might think of it by considering any checks Black can give. He has two with his queen (Qf3 and Qd8) and two with his bishop (Be7 and Bg7). If you imagine Bg7 you should see the line from the bishop through the king to the queen. Or when you consider the reply to Bg7 you realize that Black can neither interpose anything nor capture the bishop: he must move his king. When the king moves, you ask what this makes possible, especially by using the checking piece that now may have a clean line where the king used to sit. Here Black can of course play BxQ, winning the queen.