COR (Coefficient of Restitiution) is directly related to the speed of a golf ball as it rebounds from a clubface. Generally, a club with higher COR will generate greater ball velocity giving it more distance.

The Rules of Golf maintained by the USGA limits the COR on driving clubs to .830 but the rest of the world used higher COR ratings.
In terms of distance, a player with a swing speed from 100 to 120 mph should gain about two yards for every one percent increase in COR. Players with slower swing speeds will generally gain less yardage.
The compromise reached between the USGA and the R&A is that from Jan 2003 cor .860 worldwide will be adopted worlwide and elite competitions will be restricted to .830.

Previously only the US had restricted drivers to .830 for elite competitions.
Other tours have the option to choose their cor
What has also been agreed is From Jan 2008 the cor limit for drivers will be reduced to .830 worldwide

Callaway’s ERC(R) forged titanium driver will now conform under the rules of golf in the US and Callaway have stated that they will not issue any new clubs that do not conform to the new rules when they come into force.

(TaylorMade's R500 Series of drivers, including the R510, R540, and R580, boasts a COR that approaches the .830 limit.)