The Official Rules of Golf appear on the About.com Golf site courtesy of the USGA, are used with permission, and may not be reprinted without the permission of the USGA.

7-1. Before or Between Rounds
• a. Match Play
On any day of a match-play competition, a player may practice on the competition course before a round.

• b. Stroke Play
Before a round or play-off on any day of a stroke-play competition, a competitor must not practice on the competition course or test the surface of any putting green on the course by rolling a ball or roughening or scraping the surface.

When two or more rounds of a stroke-play competition are to be played over consecutive days, a competitor must not practice between those rounds on any competition course remaining to be played, or test the surface of any putting green on such course by rolling a ball or roughening or scraping the surface.

Exception: Practice putting or chipping on or near the first teeing ground before starting a round or play-off is permitted.

PENALTY FOR BREACH OF RULE 7-1b:
Disqualification.

Note: The Committee may, in the conditions of a competition (Rule 33-1), prohibit practice on the competition course on any day of a match-play competition or permit practice on the competition course or part of the course (Rule 33-2c) on any day of or between rounds of a stroke-play competition.

7-2. During Round
A player must not make a practice stroke during play of a hole. Between the play of two holes, a player must not make a practice stroke, except that he may practice putting or chipping on or near:

(a) the putting green of the hole last played,
(b) any practice putting green, or
(c) the teeing ground of the next hole to be played in the round,

provided a practice stroke is not made from a hazard and does not unduly delay play (Rule 6-7).

Strokes made in continuing the play of a hole, the result of which has been decided, are not practice strokes.

Exception: When play has been suspended by the Committee, a player may, prior to resumption of play, practice (a) as provided in this Rule, (b) anywhere other than on the competition course and (c) as otherwise permitted by the Committee.

PENALTY FOR BREACH OF RULE 7-2:
Match play — Loss of hole; Stroke play — Two strokes.
In the event of a breach between the play of two holes, the penalty applies to the next hole.

Note 1: A practice swing is not a practice stroke and may be taken at any place, provided the player does not breach the Rules.

Note 2: The Committee may, in the conditions of a competition (Rule 33-1), prohibit:
(a) practice on or near the putting green of the hole last played, and
(b) rolling a ball on the putting green of the hole last played.

©USGA, used with permission