The Fork, the pin and the skewer

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)

No. 3

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)