DivotTracker is a MacOS X application which allows you to record your golfing performance and calculate your handicap index. A handicap is a way to compare golfer's abilities, and allow for equitable competition among golfers of different abilities.
The application attempts to comply with USGA standards for handicap calculation. However, DivotTracker cannot give you an "official" USGA handicap index. You need to use a sanctioned organization to provide that. DivotTracker is only intended to give you a way to track your scores outside of that organization, or instead of an official handicap. By using the USGA handicapping rules, the handicap that DivotTracker calculates should be equal to that calculated by an official handicapping body.