The United Nations is an international organization whose purpose is to provide a forum by which nations can come together to achieve an international consensus on issues of international law and security, economic development, and human rights. As of July 14, 2011, there are 193 United Nations member states, which covers nearly all internationally recognized nation states.
In the Gargoyles Universe
In 1997, the United Nations has 184 member states. The United Nations will be the site where a delegation of the New Olympians will appear to announce their desire to restore ties with humanity.  At some point in before and up to the year 2188 the United Nations will pass the Gargoyle Minority Protection Act. 
In 2198, the Secretary-General of the United Nations will be Alexander Fox Xanatos IV, and his Chief of Staff will be Owen Burnett. The Secretary-General will be kidnapped by the Space-Spawn along with other world leaders, and replaced by someone more willing to cooperate with the invaders. 
In the Real World
The United Nations as the distinct international body it is known as today took shape in the closing months of World War II. From August to October 1944, representatives of the United States, United Kingdom, France, Republic of China, and the USSR met in Washington, D.C. to outline the goals, organization, and various organs of the emerging international body, as well as how the conceptual organization would promote international security, a critical failure of the League of Nations.
On April 25, 1945, representatives from 50 nations met at the UN Conference on International Organizations in San Francisco, California, to draft the formal United Nations Charter. The charter was signed on June 26, two months after the conference's start. Since then, the United Nations has grown to include 192 member states, and additional observer states, NGOs, and quasi-sovereign entities, effectively representing all peoples on Earth.
The stated aims of the United Nations are to prevent war, to safeguard human rights, to provide a mechanism for international law, and to promote social and economic progress, improve living standards and fight diseases. One of its first actions towards achieving these goals was to ratify the Universal Declaration of Human Rights on December 10, 1948.
The voice of the United Nations is the Secretary-General, elected by the General Assembly upon a nomination from the Security Council for renewable five-year terms. The current Secretary-General is Ban Ki-moon of South Korea, appointed January 1, 2007.
The United Nations cannot legislate for member states, and so it issues resolutions and proclamations that are advisory and represent the moral opinion of the majority of the world's nations rather than hold the traditional force of law. The United Nations does this through five organ bodies: (1) UN General Assembly, (2) UN Security Council, (3) International Court of Justice, (4) UN Economic and Social Council, and (5) UN Secretariat.
The General Assembly acts as a forum for all member states to discuss the most pressing international issues in annual sessions. Proclamations can be issued from the General Assembly either by a 2/3 majority vote, with each member state having one vote, or by consensus. The General Assembly is also the body where member states determine the course of the various inner functions and structure of the United Nations.
The President of the General Assembly is elected for a one-year term, and the position rotates among five geographic groups: African, Asian, Eastern European, Latin American/Caribbean, and Western States.
The Security Council consists of 15 member states: five permanent members (the USA, UK, France, China, and Russia) and ten members that are elected by the General Assembly every two years. The Security Council is charged with ensuring international peace and security in times of crisis and does so through the issuance of Security Council Resolutions, which may outline economic sanctions or military action against member states.
The United Nations Charter gives the Security Council broad authority to consider matters which may adversely affect international security. While any issue may conceivably be brought before the body by a majority vote, the permanent member states have veto power over the passage of any substantive resolutions which the Security Council might pass, and thus any one of those nations might prevent a resolution from being issued even if the other 14 member states agree on passage.
If the Security Council cannot agree on a binding resolution, it may issue a Presidential Statement by consensus which, while lacking the force of law, serves as a means to apply political pressure on an issue of international security.
In order to respond quickly to emerging crises, delegations of member states assigned to the Security Council must always be at the United Nations headquarters.
International Court of Justice
The International Court of Justice, located in The Hague, is responsible for adjudicating disputes among member states, although it can also hear cases related to war crimes. However, the ICJ cannot prosecute persons accused of war crimes, including genocide and crimes against humanity, a role which is reserved for the International Criminal Court beginning in 2002.
While the United Nations Charter states that all member states are automatically subject to the jurisdiction of the ICJ, member states must individually elect to become subject to the jurisdiction of the ICC. As of 2007, 104 member states have become members of the ICC, and an additional 41 member states have signed but not ratified the Rome Statute. The remaining 47 member states which have not signed on to become members of the ICC include the United States, China, and India.
Economic and Social Council
ECOSOC consists of 54 member states elected by the General Assembly for three-year terms. Its president is elected for a one-year term by mid-level member states of ECOSOC. The council's primary role is to serve as an advisory body for the General Assembly on matters of economic development and social cooperation through information gathering, although it also coordinates functions and policies among several NGOs, including the World Bank and International Monetary Fund. ECOSOC meets once a year for four weeks in July.
United Nations Secretariat
The Secretariat is an international staff that serves under the direction of the Secretary-General to coordinate the functions of the United Nations and implement the tasks and directives of the organizations various bodies.
The United Nations is perhaps best known as deploying peacekeepers to spots of particularly intense international disputes or human rights violations. All peacekeeping operations must be approved by the Security Council. The role of peacekeepers is not to engage in hostilities with member states but to, among other roles, ensure the protection of human rights and the enforcement of terms of peace laid out by the combating parties.
The United Nations peacekeeping force does not constitute a separate military branch of the United Nations, and members of any peacekeeping force retain control over their deployed military personnel. In cases where it is not feasible to deploy United Nations peacekeepers, the Security Council may authorize regional military forces to intervene on behalf of the international community, such as the North Atlantic Treaty Organization.