State Renewable Portfolio Standards (RPSs) are flexible-market based policies which ensure that public benefits of renewable energy are recognised. An RPS requires that electricity providers to obtain a minimum percentage of their power from renewable energy resources by a certain date. Each state chooses to fulfill its mandate using a combination of renewable energy sources, including wind, solar, biomass, geothermal, or other renewable sources. Some RPSs will specify the technology mix, while others leave it up to the market.
Currently there are 37 states plus the District of Columbia that have RPS requirements or goals in place. While the first RPS was established in 1983 (Iowa), the majority of states passed or strengthened their standards after 2000. One central component of an RPS is that the RPS requirement is implemented through a system of tradable renewable electricity credits (RECs). Retail electricity sellers meet the RPS requirement by either generating renewable electricity themselves or by purchasing RECs from other generators.
More detail about different State RPSs can be found at http://www.ncsl.org/research/energy/renewable-portfolio-standards.aspx