According to Appendix II, section 4c of the Rules of Golf, "The design, material and/or construction of, or any treatment to, the clubhead (which includes the club face) must not: (i) have the effect of a spring which exceeds the limit set forth in the Pendulum Test Protocol on file with the USGA, or (ii) incorporate features or technology, including, but not limited to separate springs or spring features, that have the intent of, or the effect of, unduly influencing the clubhead's spring effect, or (iii) unduly influence the movement of the ball."
This is the part of the rulebook that caused so much controversy when new rules were being developed and applied in the mid-2000-aughts: the spring-like effect. High-COR drivers, low-COR drivers, legal drivers, illegal drivers. And COR ... what exactly is "coefficient of restitution" anyway? "Characteristic time," or CT, later became a big issue, too.
What you need to know is that just as there are non-conforming golf balls, there are non-conforming drivers. And the USGA and R&A maintain lists of conforming and non-conforming drivers.
If your driver is on the non-conforming list, it means that the driver exceeds the limit for one of a number of measurements conducted by the governing bodies of new driver clubheads (primarily, the limit on COR or the CT limit).
When manufacturers create new driver designs, they submit the clubheads to the governing bodies. The USGA and/or R&A then conduct testing on the technical specs. If the driver head passes inspection, it is "legal" for use in rounds played under the Rules of Golf; if the driver's COR or CT or some other measurement exceeds the maximums under the rules, then it is "illegal" for use in rounds played under the rules.
Of course, any recreational golfer can use any driver he or she wants if he/she is ignoring the rules. (The technical limits enforced by the governing bodies are there to prevent equipment from providing what is considered an unfair advantage.)
Ready to view the lists? You can check the conforming driver list on the USGA website, and the non-conforming list on the R&A website:
(The R&A website also includes the conforming list). If you cannot find your driver on either the conforming or non-conforming list, contact one of the governing bodies.