The cue ball can be a pool player's best friend, or her worst enemy. Depending on which set of rules you're using, sinking the cue ball in a pocket (also referred to as "scratching" or "sewering") can be a minor inconvenience, a major game changer or even an automatic loss.
On the Break
Though it's rare, a player might sink the cue ball on the beak. Doing so automatically ends his turn. Even if one or more object balls were made, the table remains "open," meaning that his opponent can still choose either solids or stripes to sink. If the 8-ball falls into a pocket, the opponent can either re-spot the ball on the table and resume his turn behind the head string (the imaginary line at the near end of the table), or re-break.
During the Game
In some games, a cue ball scratch always results in the opponent shooting the ball from behind the head string, no matter when the infraction takes place. Usually, however, the penalty for scratching during the game is harsher. Under standard 8-ball rules, if a player sinks the cue ball during her turn, play is passed over to her opponent, who is allowed to place the cue ball anywhere on the table for his next shot. The cue ball may be moved as often as the player likes until he settles on where he would like to play his next shot, but it cannot be pushed forward with the cue tip. This motion counts as a played shot.
While Shooting the 8-Ball
Unless you're playing a casual game with relaxed rules, scratching on the 8-ball is the worst thing you can do. It usually means an automatic loss, even if the 8-ball is sunk in the process. Friendly games will often allow the offending player off the hook by giving his opponent cue ball in hand and forcing her to make the 8-ball, but this is rare and still provides the opposing shooter with a great chance to end the game on her own.