Handicapping

A golf handicap is a numerical measure of an amateur golfer’s playing ability. It can be used to calculate a net score from the number of strokes actually played, thus allowing players of different proficiency to play against each other on somewhat equal terms. Handicaps are administered by golf clubs or national golf associations. Exact rules relating to handicaps can vary from country to country.
A golfer’s net score is determined by subtracting the player’s handicap from the gross score (the number of strokes actually taken). The net scores of all the competing golfers are compared and (generally) the lowest score wins.

A player’s handicap is intended to show a player’s potential, not his average score, as is the common belief. A player will play to his handicap less than 25% of the time. The USGA refers to this as the “average best” method. So in a large, handicapped competition, the golfer who shoots the best with respect to his abilities and the normal variations of the score should win.

While there are many variations in detail, handicap systems are generally based on calculating an individual player’s playing ability from his recent history of rounds. Therefore, a handicap is not fixed but is regularly adjusted to increases or decreases in a player’s scoring.

A golfer whose handicap is zero is called a “scratch golfer.” A golfer whose handicap is 18 is called a “bogey golfer.” It is possible to have a handicap below 0; these are referred to as ‘plus’ handicaps, and at the end of the round, a ‘plus’ handicap golfer must add his handicap to his score. A professional golfer plays off scratch, but has no actual handicap.

In the United States, handicaps are calculated using several variables: The player’s scores from his most recent rounds, and the course rating and slope from those rounds. A “handicap differential” is calculated from the scores, using the course slope and rating, and the player’s handicap differentials are used to calculate the player’s handicap.

Information from Wikipedia

The USGA Handicap System™ enables golfers of all skill levels to compete on an equitable basis. For more about their handicap system go to http://www.usga.org/playing/handicaps/handicaps.html
Handicap systems are not used in professional golf.