Legally speaking, an international organization must be established by a treaty providing it with legal personality for it to be an international organization. International organizations so established are subjects of international law, capable of entering into agreements among themselves or with states. Thus international organizations in a legal sense are distinguished from mere groupings of states, such as the G-8 and the G-77, neither of which have been founded by treaty, though in non-legal contexts these are sometimes referred to as international organizations as well. International organizations must also be distinguished from treaties; while all international organizations are founded on a treaty, many treaties (e.g. NAFTA) do not establish an international organization, and rely purely on the parties for their administration.
International organizations can be categorised in two main ways: by their membership, and by their function.
International organizations differ in who their members are, or who is permitted to join them. Some organizations, global organizations, have membership open to all the nations of the world. This includes the United Nations and its specialized agencies. Other organizations are open to membership only within a particular region of the world. Finally, some organizations base their membership on other criteria: cultural or historical links (the Commonwealth, La Francophonie), level of economic development or type of economy (OECD, OPEC), or religion (Organization of the Islamic Conference).
List of Global organizations
List of Regional Organizations
Organizations formed on miscellaneous membership criteria
International organizations can also be considered functionally, based on the functional area in which they operate.
Financial international organizations
The Union of International Associations provides information on international organizations.