REC’s are property rights to environmental, social, and other nonpower qualities of renewable electricity generation. Once a renewable project is certified, it produces a credit for each MWh generated. These credits can be sold and/or traded in the market place bundled with or without the power generated.

Compliance Markets
Certain states require a percentage of power suppliers to provide their electricity to be obtained from renewable sources. These states have set forth renewable portfolio standards, also known as RPS requirements. States that use these RPS requirements are under compliance markets.

States that aren’t held under compliance markets belong to voluntary markets. Tshere are many different retail programs labeled as “green power” that are built by voluntary markets that are supported by government entities, companies, and consumers. Pricing in voluntary markets is varied due to the different types of facilities and their geographic location. Voluntary markets are driven by environmental disclosure, consumer demand, and corporate commitments.
(Source – DSIRE)