Chess strategy is simply the act of thinking ahead and planning your moves. In a well thought out strategy, every move is planned in advance and no move is made simply for short term gain. For example, unlike checkers, if a player exposes a piece to attack you are under no obligation to take that piece if it does not advance your strategic position. Strategy can be divided into three distinct goals, each played at different times within a chess game. They are:
The opening game. The opening game starts with the very first move and aims to progress the game quickly to a point where major pieces are protected and you have strategic advantage such as the protected acquisition of the middle board. The opening game normally consists of a memorized strategies called chess openings. The strategies must be adapted as the opponent plays their moves. Therefore there are hundreds (if not thousands) of chess openings that may be memorized and played. Recognizing the opportunity for enacting a rehearsed or memorized chess opening is key to winning the opening game.
The middle game. The middle game starts at the completion of the opening game (of course), which is normally about 10 turns in. The aim of the middle game is to gain material advantage by taking your opponents pieces. This is normally achieved through use of tactics. The middle game is also the time where strategy is enacted to progress the game to the end game. The player with the highest material advantage is not always the winner, and therefore pieces should only be taken if they bring the game closer to the end game.
The end game. The end game is where the king is directly under attack in order to bring about checkmate or stalemate if checkmate is not possible. The end game may also include tactics or strategies designed to allow pawn promotion.
Within each game, there are number of well known strategies that may be played to achieve each goal. Common strategies are learned from researching tournaments or other players at which point they enter the public domain and may therefore be used by other players. There are many online databases and video resources detailing thousands of known strategies. As a player becomes better at chess, they will acquire more strategies for use within the game.
Amongst other things, rehearsed strategies can be used for:
Breaking down an opponents defenses.
Bringing about checkmate.
Overcoming an attack.
Launching an attack.