There are three distinct stages to the game of chess that you need to know in order to be a winning chess player. These three stages are the opening, middle, and endgame. Each stage has different goals and objectives.
The opening you want to get a rapid development of your primary pieces. You also want to safe guard your king, generally by castling. It is in this phase of the game that you want to try and achieve dominance over the middle four squares of the board. Generally the opening lasts between ten to twenty moves roughly.
The middle game is when you begin to coordinate your primary pieces and attack your opponent's weak spots and open files. The goal is to win primary pieces from your opponent or even be able to checkmate your opponent. The middle game is approximately from the end of the opening phase until around move forty.
The end game is when you use your remaining primary pieces to take advantage of the weaknesses that you created in your opponent's defense during the middle game. The endgame often concludes when one of the players is able to move a pawn to the other side of the board and thus turn that pawn in for a queen. This is then followed by a checkmate or a resignation. Strategy, not tactics are what need to be considered in the end game.
Although there is no official start or end to the different stages of the game you need to have an understanding of where you should be focusing your attention. These are general guidelines as to what the stages are and goals to accomplish during each stage that successful chess players have been using throughout the history of chess.