The Union of International Associations describes an NGO as "a legally constituted organization created by private persons or organizations without participation or representation of any government...[and] are not conventional for-profit business." NGOs can be organized at any level from local to international.
A useful overview (2006): What is a Non-Governmental Organization? by Prof. Peter Willetts, City University, London
Some resources for identifying NGOs include
Examples of NGOs include
Other names for think tanks include policy institutes or research institutes.
A think tank may have an ideological perspective or may be nonpartisan. It's useful to review the About Us section of the think tank's website and look for other views about the think tank in order to place their work in context and better evaluate the authority and credibility of their work.
Note that many think tanks don't take institutional positions on topics but their researchers, scholars, or fellows may take individual, personal positions on policy and other issues, so being aware of the backgrounds of a policy report's author(s) is also helpful.
Resources for identifying or evaluating think tanks include
Examples of think tanks include