Source: Cornell University Library
An IGO is an organization composed primarily of sovereign states, or of other intergovernmental organizations. IGOs are established by treaty or other agreement that acts as a charter creating the group.
A non-governmental organization (NGO) is any nonprofit, voluntary citizens' group which is organized on a local, national, or international level. Task-oriented and driven by people with a common interest, NGOs perform a variety of service and humanitarian functions, bring citizen concerns to governments, advocate and monitor policies, and encourage political participation through provision of information.
A think tank is a research institute or organization employed to solve complex problems or predict or plan future developments, as in military, political, or social areas.