The Pin, Fork and Skewer

The Fork

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)

The Pin

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 Skewer
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)

All this I got from the following website: usefullchess.com
Click here to go to their website.

Then, a sacrifice to put the other player in a bad position - when you play again, you have forked him. (diagram 6) OR you win the game (diagram 5) 
Let's have a look:

This is an easy example. White wins after the sacrifice of a queen.
1. Qd8 + Rxd8 2. Rxd8 mate

No. 6
1. Rxf7 !! 
A sacrifice of a rook . Qxf7 2. Nd6+ !! (this is a fork. Knight attacks the Black king and queen at the same time. Black's queen is captured)