The following is a short overview of some of the more common tactical themes in chess.
There are several basic types of moves that win material.
A fork occurs when a player makes a move that attacks two of his opponent's pieces simultaneously with one piece of his own. This move forces his opponent to choose which piece he will lose. Knights are especially adept at forking because their method of movement allows them to attack any other kind of piece without being under attack from that piece (unless, of course, that other piece is also a Knight). Forks in which the enemy King is one of the pieces under attack are especially effective because they force the opponent to move their King, leaving no choice as to which piece to move. While the Knight is the most adept at forking, all of the other pieces are capable of it.
A player creates a pin when makes he attacks an opponent's piece in between his attacking piece and another more valuable enemy piece. The opponent cannot move the attacked piece without losing the more valuable piece behind it, thus effectively "pinning" the piece. Pinning pieces to the enemy King are most effective because the pinned piece cannot legally be moved; if it was, the King would be exposed to check.
The skewer is the opposite of the pin. It occurs when a player makes a move such that an opponent's valuable piece is directly between his attacking piece and another of the opponent's less valuable pieces. The skewer forces the opponent to move his more valuable piece out of the way, which allows the skewering player to capture the less valuable piece behind it. As with the fork and pin, skewers are especially effective when the King is the skewered piece because the it leaves the opponent with no choice.
The Discovered Attack
A discovered attack can occur in positions where a player has a Bishop, Rook, or Queen behind any other piece. When the intervening piece (the piece in front of the Bishop, Rook, or Queen) moves, it uncovers the power of the piece behind it, potentially attacking an opponent's piece in the process. Discovered attacks can be devastating if the piece being moved also attacks an enemy piece, thereby attacking two pieces with one move. As with the fork, the opponent may be forced to accept the loss of a piece, no matter what he does.
A sacrifice occurs when a player intentionally allows one of his own pieces to be captured in exchange for a better position, later material gain, or even checkmate. Often, the piece is offered in an attempt to get the opponent to weaken his own defenses by capturing with a piece that had been in a good defensive position. Morphy was well known for his games featuring many sacrifices, as was Mikhail Tal, a Latvian who was briefly world champion in the 1960s.