The stop shot is one of several weapons in the pool shooter's bag of tricks. It has a few different uses, and is definitely a shot that an aspiring pool player will want to know.
The stop is done by hitting the cue ball at dead center with the cue tip. The cue is hit fairly hard, so that it skids across the cloth instead of rolling. If done correctly, the cue ball will stop moving right where it contacts the object ball.
This will work for the shorter shots, but if you are attempting the stop shot on a length-of-table shot, a hit slightly below dead center will work better. The backward spin that this will cause on the cue ball, will help prevent the ball from rolling on the cloth, before it reaches the object ball.
One of the most obvious times that you will want to use the stop is when the object ball is right at the edge of the pocket. If the cue ball is hit normally, it may follow the object ball into the pocket and scratch. The stop will prevent this from happening.
This shot is also used when you wish to leave the cue ball in a certain location for the next shot. Sometimes several shots are lined up near each other, and by using the stop on them, easy, short shots can be made.
The stop is also good for keeping the cue ball in a particular section of the table. You may have several balls all at one end of the table, and if the cue ball motion isn't curtailed, it may end up down the other end of the table and leave you nothing to shoot at next.
The following video shows three different types of shots - the follow, stop, and draw. These three shots are the basic shots that every pool player should master, to have at least some ability to control the cue ball position to his advantage during a pool game.