A fork is a tactical theme in chess. It occurs when one moves a piece so that it attacks two pieces at once. One's opponent can only move one of the pieces, so the other one is easy pickings. Here is a game with a sample fork:

1. e4 e5
2. Nf3 Nc6
3. Bc4 Bc5
4. Ng5 d6?
5. Nxf7

This is a knight fork. The knight simultaneously attacks both the queen and the rook. When black moves one, white can still capture the other. However, this does not mean more powerful threats cannot be made. Let's see what happens if white gets greedy.

5. .. Qf6
6. Nxh8 Qxf7#