The Rules of Golf regulate both game play and the equipment used to play golf. Rule 4 and Appendix II outline the rules as they apply to all clubs, including drivers. Disobeying Rule 4 can result in penalties, but you also need to be aware of Appendix II. It's aimed primarily at manufacturers, but companies do build non-conforming clubs for weekend players. Knowing the basics can keep you from accidentally buying a club you can't use in competition.
Drivers must conform to most of the same rules as the other clubs. For example, drivers need to be "traditional," with only have one head and one shaft. It's possible, however, to have a club without material added for a grip. Lead tape may be used to adjust the swingweight as a concession to tradition. Although the rules have been amended to allow adjustments to help with custom fitting, the rules prohibit club materials that move during use or can be easily changed during play. A club's shaft length must be greater than 18 inches but not more than 48 inches.
The driver's clubhead is where most manufacturers become experimental, so it's also regulated. You can't have holes of any type in the clubhead – apart from the shaft's mounting hole, of course – and protrusions or depressions in the head's body must not extend onto the face. The head must be longer from heel to toe than from face to back. It can't be longer from heel to toe than 5 inches or taller from sole to crown than 2.8 inches. And the volume of the clubhead can't be more than 460 cubic centimeters, with an allowed variance of 10 cubic centimeters.
The clubface must be rigid and smooth. Modern clubheads are made from many materials, but the rules use a traditional steel face as a standard. Any face that creates significantly more or less spin than a standard steel face or acts like a trampoline to "bounce" the ball a greater distance is not allowed. Faces that are grooved or "punch-marked" are allowed, but rules regulate the size and depth of these marks.
Rule 4 of the Rules of Golf regulates a driver's use during a round. You can't purposely change or adjust a club in any way; if you do, it becomes a nonconforming driver and you face a penalty. If you carry the nonconforming driver but don't use it, the penalty is two strokes per hole (to a maximum of four strokes) in stroke play or loss of the hole (to a maximum of two holes) in match play. If you use it in either format, you're disqualified. However, if you have a conforming driver that becomes damaged in the course of normal play, you may continue to use it without penalty.