On Certified Emission Reduction
Once property rights are defined, even pollution rights could be traded. Carbon emissions are now actively traded between developed countries and developing countries. 
Basically, the rich countries signed a scheme called Clean Development Mechanism, or CDM, in which they agree on an emission reduction target to be achieved within a few years. However, for these rich countries to reduce emission is very costly because the existing ways of production or ways of life are hard to change. So the UN allows them to buy emission quotas from other countries, typically developing countries that are rapidly building up production lines and accumulating wealth, to partially fulfill their obligations. Effectively, developing countries that have found a way to reduce emissions will be rewarded in the form of getting paid for the emission reduced. So there is a stronger incentive for developing countries to develop cleaner projects because the extra cleanliness can now be monetized. 
How Are CERs Measured?
The quantity of emission reduced is certified by UN and registered in units of CER credit. One unit of CER credit represents the equivalent to one tonne of carbon dioxide reduced under CDM. How are CERs measured? 

CER in China After 2012
Currently China is the largest CER seller in the world because it is not an Anex I country for the moment and does not have an obligation  to reduce emissoin. But as the current treaty expires in 2012, will China be included as well? 
The Price Behavior of CERs in Primary and Secondary MarketCERs are considered a commodity in many senses. It has a primary market where CERs are orginated. This is the place project owners of clean energy would sell their CERs to an intermediary. It also has a secondary market where CERs are traded and bought by the final users, typically producers from developed nations.  References:Wikipedia.org
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