A Pin is a tactic used to freeze an opponent's piece so that it cannot be moved, either because moving it will cause the king to be captured, or will cause the loss of a major piece, such as a queen.  A Pin has some advantages, the chief among them being the immobilization of the piece being pinned.

Discovered Check

The tactic creates havoc with the enemy pieces and often leads to the loss of material of consequence. It occurs when the piece that can check the king is blocked by another piece of the same color. The removal of the blocked piece to another square then puts the king in check. Either the king has to move, or another piece must be interposed between the opposing piece checking the king. In any case, the moved piece that caused the discovered check can go on to take material.

 

Knight Fork

A knight always attacks a piece or pawn in a different colored square from the one it stands on. Thus, if a knight is on a dark square, it can only attack an opposing chessman that is on a white or light square. Sometimes, if a knight is well-positioned, it can attack two enemy pieces at the same time, when both pieces are on the same colored square. When this happens, one of the pieces must be captured, for it cannot evade capture by the interposition of a piece between the knight and the piece to be captured.

Double Check

This tactic occurs when two pieces check the king at the same time.

Combinations

Combinations can be said to be the "soul of chess". A combination is a series of moves, thought out in advance, in which several moves are forced on the defender, who has no choice in the matter, but which leads either to the defender's loss of material or mate. Often these combinations involve the sacrifice of material by the attacker, and often the defender doesn't realize that in the end it will be he who will suffer, not the attacker. The great players in history have all been masters of the combination, and it is one of the essential ingredients in any fine player's makeup. To see ahead, to plan several moves ahead, to sacrifice material for the purpose of winning the game -these traits are to be hailed. It is what separates the great from the good players. And it is a joy often to replay games in which combinations have won the day.