Fork
A fork is a situation where one of your pieces simultaneously attacks two of your opponent's pieces. Because the opponent's can't save both pieces at the same time, your attacking piece is guaranteed to capture one of your opponent's pieces.
Diagram A
In Diagram A, Black's knight has forked White's rook and bishop. If White saves his rook with △R-74, then Black plays ▲Nx44, taking White's bishop. If △B-55, then ▲Nx64 wins the rook.
Diagram B
There is a fork in Diagram B. Because of Black's 5f pawn, he can fork White's pieces with ▲S*55. White can recapture by △Bx, but ▲Px takes the bishop. Even if the forking piece can be recaptured, as long as it is protected by another piece, it can still create a fork.
Diagram C
Black can fork the golds in Diagram C with ▲R*66 or ▲R*44.
There are some situations that look like a fork, but in the end, both pieces can be saved.
Diagram D
In Diagram D, it appears as though White's knight has forked Black's rook and bishop, but Black has ▲R-18, delivering check. After the king removes the check, Black can save the bishop.
