If you attack two of the opponent's pieces at the same time by using one piece , this is called a fork.
A White Bishop attacks to a Black King and Knight. Black is obliged to lose his Knight.(Diagram 1)
A White Knight attacks to a Black Queen and Rook. Black is obliged to lose one of its pieces.
A Queen and a pawn can also fork the opponents (see Diagram 2)
Black Knight can't move because it will lose his King. (see diagram 3) This position is called as a pin.
The Black Bishop is also pinned by a White Queen.
The White Rook is attacking to more valuable black queen .
The Queen must escape but the Black Rook will be lost. This is called a skewer. (see Diagram 4)