An intergovernmental organization (IGO) is a group of members (sovereign states, countries) working together on issues of mutual self-interest.
Some IGOs are legal entities – i.e. they have been ratified by charter. The United Nations is a good example of a legally constituted IGO. Its charter links the interests of 192 member nations.
IGOs that are not chartered do not “exist” in a legal sense. But in other significant ways they certainly do exist. For example the non-chartered G7 (Group of Seven) has annual meetings dealing with the substantive political and economic concerns of the world's industrial leaders (the G7 was the G8 until 2014, when seven members suspended Russia for its annexation of Crimea).
The former Organization of African Unity is comprised of 54 of the continent's states, and has a vast slate of interests and objectives: solidarity, defense, peace, human rights, development, education, poverty, inter-state economic integration, etc.
Use “Select Language”. A 22-member group of states concerned with the region's economic improvement, dispute resolution, and coordination of political interests
Emblem of the Arab League
21 Pacific-rim countries aiming at economic cooperation, free/freer trade, and sustainable growth
Commonwealth of Nations is a 53-member association of former British-empire territories. Objectives and interests include peace, democracy, individual liberty, free trade, equality, and development
Regional economic cooperation, with a stated goal of a political federation of states
Political-economic union of 28 member states
The ICC is an IGO and international tribunal that tries cases of war crimes, genocide, and crimes against humanity
International organization of police forces. Political Handbook of the World identifies Interpol as an IGO, but Wikipedia calls it an NGO. If you define an IGO as a group of nations, then Interpol is arguably an NGO. Or it might have qualities of both
IDB is the largest source of development financing in Latin America and the Caribbean
Finances infrastructure, modernisation, and education projects in its member states and non-member Muslim countries
OPEC (Organization of the Petroleum Exporting Countries)
A cartel of oil-states with the fairly limited aim of coordinating and unifying the oil production policies of its members
15 states in Africa that want improved social & economic cooperation, political integration, and enhanced security
The largest IGO, it deals with issues of peace, security, development, & human rights
Both an international organization and a military union
The successor organization to GATT regulates and supervises the international trade of its members
Other resources on IGOs:
IGO library guide at Northwestern University has an extensive alphabetical list of IGOs, including tabs for the agencies of the UN and European Union
Harvard Law School page defining and describing IGOs.