Renewable Energy Certifications (typically refered to as "RECs", and also known as "green tags" and "tradable renewable certificates") are at the heart of sales of renewable electricity. For more detail on what they are and how they work, please use the resources below.
For a simple description of Renewable Energy Certificates and an illustration of how they are created, see the listing in the Green-e Dictionary.
The former president of Center for Resource Solutions (CRS), Jan Hamrin, has written an excellent primer on RECs, called REC Questions and Answers. It contains information on the definition of a REC, how the REC system began, how RECs are created, tracked and verified, and how pricing works.
Each year, CRS collaborates to put on the Renewable Energy Markets conference. As part of a pre-conference 101 session, renewable energy market participants hold a Renewable Energy 101 session. For the most recent set of slideshows from this session, see the first section of the agenda from the last conference. Four slideshows are available, covering the basic terminology of renewables, data and growth trends, market oversight and operation, and climate change.