In the skewer two pieces in a line are attacked by a queen, rook or bishop. A skewer is very similar to the pin, but in case of the skewer the most valuable piece is the front piece. So instead of being pinned the piece has to step out of the way allowing the piece behind it to be captured.
The front piece is often a king, but it may be another piece.

In the following diagram white can administer a skewer by 1.Bc4+.

E.g. 1…Kg7, after which white can capture the queen 2.Bxg8.

To spot possible skewers (and pins) you first have to look for valuable pieces on the same diagonal, line or file. The second step is to find the right attacking piece.

 

After playing 1.Be4 in the diagram on the left the resulting position can be called a pin or a skewer, because the value of the front piece is the same as the value of the back piece.

The name doesn’t matter either. 

After 1…Rbb6 2. Bxc6 Rxc6 white has gained a rook for a bishop.
 

At first sight the third diagram seems to have a possibility for a skewer. But this time white doesn’t gain anything by the move 1.Be4.