Carbon markets are trading systems where carbon is quantified in "carbon credits" that can be bought and sold. Companies or individuals can use carbon markets to offset their greenhouse gas emissions by purchasing carbon credits from entities that remove or reduce greenhouse gas emissions.
One traded carbon credit is equal to one ton of carbon dioxide or an equivalent amount of reduction, storage, or avoidance of other greenhouse gases. When the credit is used for emission reduction, storage, or avoidance, it becomes a offset and is no longer tradable.