According to Harvard Law School, "The term intergovernmental organization (IGO) refers to an entity created by treaty, involving two or more nations, to work in good faith, on issues of common interest. In the absence of a treaty an IGO does not exist in the legal sense. For example, the G8 is a group of eight nations that have annual economic and political summits. IGOs that are formed by treaties are more advantageous than a mere grouping of nations because they are subject to international law and have the ability to enter into enforceable agreements among themselves or with states." For more in-depth definitions and descriptions, consult IGO-NGO Cooperation from Duke University Libraries.
According to Harvard Law School, "IGOs are distinguishable from nongovernmental organizations (NGOs) in that NGOs are formed by two or more individuals rather than by nations. NGOs are therefore typically independent of governments, are usually non-profit organizations, and receive at least a portion of their funding from private sources. IGOs have the financial and political support of its members." For more in-depth definitions and descriptions, consult IGO-NGO Cooperation from Duke University Libraries.
Like all sources you encounter, be sure to evaluate the relevance, quality, and perspective of works put out by think tanks and research institutes.
Evaluating Resources from the University of California Berkeley clearly presents key questions you want to ask of your sources: