Fork- A fork is a move that uses one piece to attack two or more of the opponent's pieces simultaneously, hoping to achieve material advantage, since the opponent can only counter one of the two threats.
Pin- A pin is a move which forces one of the opponent's pieces to stay put because moving it would expose a more valuable piece behind it.
Skewer- A skewer is a move which attacks two pieces in a line, similar to a pin, except that the enemy piece of greater value is in front of the piece of lesser value.
Discovered Attack- A discovered attack is a move which unmasks an attack by another piece. A piece is moved away so as to unmask the attack of a friendly bishop, rook or queen on an enemy piece.
Sacrifices- Sacrificing some material is often necessary to throw the opponent's position out of balance. The sacrificed material is sometimes able to be regained with interest a couple of moves later. Pawn sacrifices in the opening are known as gambits; they are usually not intended for a short-term gain, but instead to achieve a more active position.
Zugzwang- Zugzwang (German for compulsion to move) occurs when a player is forced to make an undesirable move. The player is put at a disadvantage because he would prefer to pass and make no move, but a move has to be made, all of which weaken his position.
Zwischenzug- Zwischenzug (German for intermediate move) is a common tactic which, instead of countering a direct threat, a move is played which poses an even more devastating threat, often an attack against the queen or the king. The opponent has to counter that threat first, and this will ideally change the situation to a disadvantage for that player.