In chess clubs, chess tournaments and chess matches a game clock is used by both players to measure time. The chess clock consists of two adjacent clocks. When you have made your move you press a button with the same hand that made the move to stop your clock, while starting the clock of the opponent.
Each clock measures the total time each player takes for his or her moves. The time limit in tournament chess is usually 2 hours for 40 moves for each player. After that you might get an additional 60 minutes for the rest of the game. This all depends where you play.
It is a good idea to move quickly in the beginning of the game and play a memorized opening variation using up just a few mimutes for 12 moves or so to save time.
You surely will need more time later on in the late middlegame or endgame when the position usually gets more complicated. Then you are more relaxed when you have plenty of time to ponder about difficult moves.
When you ran out of time you have lost immediately. Even if you have a winning position this doesn't matter. This can be quite annoying when you have worked hard for hours to build a winning game and then lose on time.
Some players always run into time trouble and have to play quickly in the end. To be in constant time trouble puts stress on the mind. So make sure you keep making your moves regularly and don't use too much time for a single move.
A chess clock shows how much time both players still have left and when each player has run out of time. When your time runs out your “flag” falls and your opponent should claim a win on time immediately.
Make your move, then press the button on your side and your opponent's time starts to run. When he makes his move, he will press the button on his side which stops his time and your time starts to run.
How to set up the Clock? Let's say you play a thirty minutes game with somebody. (Rapid chess) This means every player will have a total of 15 minutes for the WHOLE game. The total game will last 30 minutes. Set both timers to read 5:45. This way your time expires at 6:00 and then your red flag will fall. If your opponent thinks longer and uses up more time his flag will fall earlier than yours.