Each driver starts with 12 points on their license, and loses points for violations. Exceeding the speed limit by 20 kph or less has a two-point penalty, for example. Once you get to zero, your license is automatically suspended for six months.
But if you get a traffic citation, you can pay 300-1500 euros per point to someone who is willing to take the rap for you (either because they don't drive or are sufficiently far from zero that the penalty won't bother them), and they'll incur the points by sending in their information on your ticket. The French Interior Ministry is attempting to investigate means to crack down on this, but the volume of tickets is apparently making it difficult.
More at the Reason blog. I think this mechanism could work well for photo radar speeding tickets in the U.S.