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."The governing bodies of golf - the R&A and the USGA - rule on the conformity of golf tees, same as they do for any other golf equipment.
Modern golf tees are pegs that are pushed into the ground, usually made of wood or plastic/rubber compounds. Typically, the top end of the tee is flared and concave to support the golf ball and keep it stable and stationary; however, the design of the top of the peg can vary.
Tees may only be used when playing the first stroke of a hole from the teeing ground. An exception is when there is a penalty that requires the golfer to return to the teeing ground and replay the stroke.
How high should you tee the ball? The depends on what club you're using.