Fork (chess)

In chess, a fork is a tactic in which a piece attacks two or more pieces simultaneously. The attacker usually aims to capture one of the forked pieces, or threaten checkmate. The defender often finds it difficult to counter two or more threats in a single move. 
A fork is most effective when it is forcing, such as when the king is put in check.
Apart from pawns on an outside file, any piece can fork, but knight forks are the most striking, since the knight is a minor piece and can be traded, and because, except in the case of other knights, a knight cannot be captured by the piece it is threatening.
Since the queen is usually more valuable than the pieces it attacks, a queen fork gains material only when the pieces attacked are undefended or if one of them is the king and the other is undefended.
Terminology.
A fork is also known as a double attack. The type of fork is named after the type of forking piece. For example, a knight fork. The attacked pieces are "forked". If the King is one of the attacked pieces, the term "absolute fork" is sometimes used. A fork not involving the enemy king is in contrast a "relative fork".
A fork of the king and queen, the highest material-gaining fork possible, is sometimes called a "royal fork". A fork of the enemy king, queen, and one (or both) rooks is sometimes called a "grand fork". A knight fork of the enemy king, queen, and possibly other pieces is sometimes called a "family fork" or "family check".
Game examples.
This example is from the first round of the FIDE World Chess Championship 2004 between Mohamed Tissir and Alexey Dreev. After 
White resigned. In the final position the black knight forks White's queen and rook; after the queen moves away, Black will win the exchange.
This example is from the ninth round of the Clarin GP Final between Guillermo Soppe and Fernando Braga. After 
White resigned. The only move is 41.Ke2 which enables a royal fork with 41...Nc3+, winning the queen.
In the Two Knights Defense (1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6) after 4.Nc3, Black can eliminate White's e4-pawn immediately with 
due to the "fork trick" 
regaining either the bishop or the knight. 