How does a cap-and-trade system work?

In a cap and trade system emitters are limited to the tonnage of pollutants they can emit. If they go higher they must pay for the excess emissions. Some emitters though are lower than the permitted emissions and can sell the "room" between their limits and their actual emissions to emitters who are over their limit. This is usually cheaper for the high emitters than the construction of treatment systems or paying thee government penalty.Many people feel this is not suitable system as it offers no incentive to the high pollution sites to improve.