Ideally, your golf score is nothing more than the number of shots it took you to complete your round. But there are plenty of rules in place that can add to your score, such as penalty strokes for hitting the ball out of bounds or into water hazards. If it's a casual golf round among friends, you might not follow these rules to the letter, but if you're ever in competition, you'll want to know how to make sure your final scorecard is official.
Every shot counts
From your tee shot to the putt that sinks the ball in the hole, you must count every shot, even if it's an accidental tap of the ball on the green or five shots to get out of a sand trap.
Out of bounds
If you hit your ball beyond the white stakes that separate the playing area from out-of-bounds, you must take a penalty stroke and re-hit your shot from its original location.
Signing the scorecard
This one only really applies to professionals in tournaments, but golf has very specific rules about the importance of adding up your score correctly at the end of the round and signing your scorecard. Failure to do so can mean disqualification.
Hitting into a water hazard is also a one-stroke penalty, but where you play your next shot from depends on where you are on the hole. If you're in line between the tee and the hole, you can drop your ball anywhere as long as it is not closer to the hole than where your errant shot landed. If you hit into a lateral water hazard--that is one not in line between the tee and the hole, you take the penalty stroke and drop the ball two club lengths from the hazard, again no closer to the hole than your original shot.
If you move a loose impediment within a club length of your ball, such as a twig or leaf, and your ball moves, you must take a one-stroke penalty and place the ball in its original location.
If you lose a ball (and it is considered lost if you cannot find it within five minutes of looking), you must take a penalty stroke and play your next shot from where you hit your last shot.
The golfer with the lowest score on the preceding hole tees off first on the next hole. This isn't so much a rule as a tradition that is employed on the pro tour as well as the local links. In a tie, allow the person who teed off first on the last hole to continue doing so. In some tournament formats, if you play out of turn, your playing partner can make you re-hit your last shot.