A skewer, sometimes referred to as an x-ray attack, is sort of a mirror-image pin. It requires a long-range piece and two enemy pieces on the same line of attack, just like a pin does, but in a skewer the more important or powerful piece is doing the shielding. Therefore, instead of immobilizing the piece, a skewer practically forces it to move, thus exposing the poor little guy it had shielded. When the two enemy pieces are of the same value, it is also referred to as a skewer, rather than a pin.
Pins and skewers are nothing more than specialized forms of double attack. It's just that both enemy pieces stand on the same line, so the attacking is being done through one piece on to the other.
Here is an example of a skewer in action:
This position, with Black to move, came up at a regional tournament in the Midwest during the mid-1990s.
The player handling the Black pieces didn't notice a short combination leading to a skewer, and so failed to defeat the chess master he was playing.
First, notice that White threatens to win a rook, since he has a battery of rooks on the d-file. Black can move his rook out of the way with 1…. Rdc8 or exchange rooks. Here's what happens if he exchanges rooks:
1…. Rxd2 2. Rxd2
Do you see the skewer now? Simply look for any two White pieces on the same line as one of your long-range pieces (you've only got two), and you will see it.
Here it is. The rook is more powerful that the knight. If it moves out of the way, Black will simply capture the knight:
3. Rd7 Bxg5.
The best defense is to let the rook go with 3. Nf3. That way, White at least gets the bishop for the rook after 3…. Bxd2 4. Nxd2.
There are many kinds of tactical setups that require more than two pieces. These include convergence and batteries (two friendly pieces going after an enemy), discovered attacks of various sorts (two friendly pieces going after more than one enemy), forks (one piece going after more than one enemy), and pins and skewers (one piece going after two enemies). These are part of your arsenal of weapons in chess.