An Inter-governmental Organization (IGO) is defined as "association of States established by and based upon a treaty, which pursues common aims and which has its own special organs to fulfill particular functions within the organization."
Encyclopedia of Public International Law. IGOs are established by treaty or other agreement. The founding treaty or agreement acts as a charter for the organization. The legal authority and structure of the IGO can be found in its charter.
The founding charter may be annexed by additional treaties or agreements if the functions of the organization grow over time. An IGO is provided privileges and immunities in pursuit of its mandate, and may be global (the United Nations) or regional (ASEAN) in scope. Privileges and immunities include exemption from taxes, customs duties, inviolability of premises and documents, and immunity from judicial process.
Hallmarks of an IGO:
The founding charter and later agreements that affect the structure of the organization can typically be found at that organization’s web site. Documents and reports may be published directly by the IGO, or in other IGO publications (such as the United Nations or the Organization of American States). You can review their web sites to make that determination. Many well known international organizations are actually subsidiary bodies of the United Nations, and may collect national and international laws related to their mission. For example, the International Labor Organization is a specialized agency of the United Nations. At its web site, you can find English translations of national labor legislation and related treaties regarding the treatment of workers.
You can review the Directory of these U.N. sponsored organizations at the U.N. web site.
Google search for IGOs:
Thte Union of International Association maintains a searchable directory of international organizations at http://www.uia.be/s/or/en/igo
The International Development Organization lists IGOs and also archives many of their publications.
The CIA maintains a list of International Organizations.
You can also consult the subscription to the Encyclopedia of Associations via the UH library system.
A Non-governmental Organization (NGO) is defined as “those organizations founded by private individuals, which are independent of States, oriented towards the rule of law, pursue public rather than private goals as an objective, and possess a minimal organizational structure. - Encyclopedia of Public International Law.
NGOs are not established by treaty. They do not have the same privileges and immunity status as an IGO; however, like an IGO, they can be global (Human Rights Watch) or in scope. Privileges and immunities include exemption from taxes, customs duties, inviolability of premises and documents, and immunity from judicial process.
NGO’s and the people who organize them are committed to a particular purpose and can be large organizations or small groups. Their commitment often results in policy positions, reports, and collection of national, customary and international laws that are important to their cause. For example, the International Committee of the Red Cross maintains an excellent library of international and customary law on the laws of war and peace. (The ICRC does not exactly sit in IGO or NGO category. It was not formed by treaty and therefore does not meet the standard definition of an IGO.)
Not all NGOs are created equally. Article 71 of the United Nations Charter provides opportunities for NGOs to obtain consultative status with the U.N. NGOs interact with the UN Secretariat, the Economic and Social Council, programs, funds, agencies, and UN Member States. Organizations with consultative status have greater access to intergovernmental meetings at the U.N. and with the U.N. subsidiary organs, and may have the ability to hold summits and conferences on matters of international law. These NGOs are highly developed and are likely to provide more research materials and make their libraries available than those organizations without consultative status.
For more information about the consultative status with the U.N. Economic and Social Council visit the Ecosoc web site on consultative status.
The September 2013 list of NGOs with ECOSOC consultative status is published in E/2013/INF/6 - (Note that each organ of the U.N. will have its own list of NGOs with consultative status for that U.N. body.)
Google search for NGOs:
World Association of Non-Governmental Organizations (WANGO) hosts a searchable database of NGOs.
The CIA maintains a list of International Organizations.
NGO Café is a meta directory of NGO databases and directories.
You can also consult the Encyclopedia of Associations - a subscription based reference work - via the UH library system.