Think Tanks can fall into a variety of categories. The following categories are from the University of Pennsylvania's Think Tanks and Civil Societies Program.
AUTONOMOUS AND INDEPENDENT Significant independence from any one interest group or donor and autonomous in its operation and funding from government.
QUASI INDEPENDENT Autonomous from government but controlled by an interest group, donor, or contracting agency that provides a majority of the funding and has significant influence over operations of the think tank.
GOVERNMENT AFFILIATED A part of the formal structure of government.
QUASI GOVERNMENTAL Funded exclusively by government grants and contracts but not a part of the formal structure of government.
UNIVERSITY AFFILIATED A policy research center at a university.
POLITICAL PARTY AFFILIATED Formally affiliated with a political party.
CORPORATE (FOR PROFIT) A for-profit public policy research organization, affiliated with a corporation or merely operating on a for-profit basis