A golf tee is typically a thin, wood or plastic peg, two or three inches in height, atop which a golf ball sits in a stable and stationary position. The tee is pushed down into the turf on the teeing ground, leaving a portion of the tee above ground, and the ball placed atop the golf tee prior to playing the stroke. A golf tee may only be used on the teeing ground under the rules, although the use of a tee is not required. How high the tee lifts the ball off the ground is up to the golfer (although the length of the tee plays a key role in that, obviously) and depends on various factors such as the club being used for the stroke.
In the Official Rules of Golf, "tee" is defined thusly:
"A 'tee' is a device designed to raise the ball off the ground. It must not be longer than 4 inches (101.6 mm), and it must not be designed or manufactured in such a way that it could indicate the line of play or influence the movement of the ball."
Tees are mentioned throughout the Rules of Golf, but in particular in Rule 11 (Teeing Ground).
For more about the golf tee, see: