In chess, a fork is a move that uses one piece to attack two of the opponent's pieces at the same time, hoping to achieve material advantage because the opponent can only counter one of the two threats. Knights are often used for forks: they jump to a position from where they attack two pieces. In the diagram to the left, the white knight is forking the black king and rook. It is particularly effective to fork a king: the rules require immediate attention to a threat to the king. In this situation, black cannot choose to defend another piece, he cannot make an intermediate move to complicate the situation; he must move the king, after which white can capture the rook.
Pawns can also fork enemy pieces: by moving a pawn forward, it may attack two pieces: one diagonally to the left and one diagonally to the right.In the diagram, the black pawn is forking the two white rooks. (Remember that by convention the board is oriented with black's first row at the top, so the black pawn is moving downward.)
A queen move also often attacks two pieces at the same time, but this is only useful if both pieces are undefended. Since the queen is more valuable than the pieces it is attacking, it is usually only profitable for it to capture undefended pieces.
A "royal fork" is sometimes used to describe the situation where the King and Queen are forked.