There are two very different models to effect major change in output of carbon dioxide by countries and companies into our atmosphere. One approach is to use the open market concept, and allow companies that effect significant reductions in carbon emissions to offer a "credit" to be purchased by companies that would find it difficult or impossible to make similar changes in their own company. The impact of this is to make it financially beneficial to those companies willing to make a change. This will make it more likely that companies will find it possible to make the investments. There are also programs where companies can find an opportunity to "buy" carbon credits from companies that invest in growing trees to offset the carbon output. This benefit goes mostly to those willing to move quickly, either for financial or marketing reasons. The enforcement behind this is to require companies to "reduce" their carbon output either by remediation efforts, or to purchase credits from those that do make these efforts.
Carbon credit is the right to emit one tone of carbon dioxide. This right or permit is usually awarded to countries that have a green house with reduced emissions. Read more details at http://www.carboncreditmart.com/.
Carbon credits are a way of allowing companies to buy and sell the right to emit carbon dioxide into the atmosphere. The schemes are highly complex, but are theoretically meant to limit CO2 emissions while limiting the impact of regulation on the economy (it allows those companies that derive the greatest profit from polluting to continue doing so). In practice, it allows corrupt politicians to grant favors to their corporate supporters, and derivatives traders to profit in a new (and very artificial) market. It's not clear that they actually reduce CO2 emissions, but they have made some people rich.