Rules of Amateur Status
DEFINITION OF AMATEUR GOLFER:
An amateur golfer is one who plays the game as a non-remunerative or non-profit making sport and who does not receive remuneration for teaching golf or for other activities because of golf skill or reputation, except as provided in the Rules.
 
There are several acts which are contrary to the definition of an amateur golfer and can cause forfeiture of amateur status.  These are outlined start of page 119 of the Rules of Golf book.  Of these, playing for "Prizes" is probably the most critical for the average golfer to understand.  Rules 3-1 describes prizes which are considered in violation of the Rules of Amateur Status.
RULE 3-1: PLAYING FOR PRIZE MONEY
An amateur golfer must not play golf for prize money (i.e., cash, regardless of amount) or it's equivalent in a match, tournament or exhibition.  Note: A player may participate in an event in which prize money (cash) or it's equivalent is offered, provided that prior to participation, she irrevocably waives her right to accept prize money in that event.  (See USGA Policy on Gambling for explanation of playing for prize money.
 
RULE 3-2:  PRIZE LIMITS
An amateur golfer must not:
* The USGA increased this limit from $500 to $750 in 2004.

For more information see: Rules of Amateur Status in the USGA Rules of Golf Book or www.rulesofgolf.com.