A think tank is "an organization, institute, corporation, or group that conducts research and engages in advocacy in public policy." (Sourcewatch.org).
Most are non-profit, but some are funded by governments, advocacy groups, or corporations, and may derive revenue from consulting or research work related to their projects. They can be non-partisan or partisan, so pay attention to the goals of the organization when using the information they generate.
When using information from a think tank:
Maintained by the Center for Media and Democracy. A good place to find additional information about a think tank.
Use to get an overview of the issues that a think tank is discussing and what social, policy or political biases that it brings to the discussion. Be sure to view the history page to determine if there are disputes about the information on the page
Search for the think tank in major newspapers like the New York Times or Washington Post to see if their research is covered in the news. Also look to see how the newspapers characterize the organization. For example, the Center for American Progress is often described as a "left-leaning organization" while the American Enterprise Institute is described as a "conservative research organization."