A chess clock allows you and your opponent to play with time control. First you agree on a certain time maximum for each player, then you set the clock - and the game can start! But beware - once you´re run out of time, you loose the game, no matter of how good your pieces may look like.
This adds a lot of thrill to a chessgame and fosters your ability to take decisions more intuitively and in a shorter time. And it enables you to have quick games, for example when you feel like playing chess without spending too much time on it.
The "DGT FIDE 2010" is a classic used preferably in tournament chess and by the World Chess Federation (FIDE). It allows you to choose from a wide range of game modes, and its menu is very user-friendly.