Think Tanks- "...an imprecise phrase used to describe a wide range of non-profit research organizations which engage in public policy analysis and research, and may/often advocate solutions." ( Lehman Social Sciences Library, Columbia University). Examples include Urban Institute, Brookings Institute, etc. See next definition as well.
Other Related Terms: Research Institutes, Policy Research Institutes, Foundations-organizations where staff conduct economic and social policy research that is "non-partisan," i.e., not politically affiliated. However, this does not mean the organization does not have a perspective on the issues, but it may be less directly aimed at getting a specific legislative or regulatory result. Foundations may also fund the research of others, e.g., Commonwealth Fund. And, there are research institutes affiliated with universities.
Interest Groups, Advocacy Groups-a collection of individuals who seek joint ends through political action and tend to have narrow goals focused around an issue or issue area. These can be: private groups, sponsored by corporations, private industry; non-profit or citizens groups, e.g., AARP, Children's Defense Fund; labor groups. The organizations may conduct research or use others research to support position, provide policy briefings, etc.
Bottom Line: Make sure you read the "about us" or mission statements of the organizations to understand what you are seeing! Context is important!