Organizational structure

The United Nations system is based on five principal organs (formerly six - the Trusteeship Council suspended operations in 1994); The General Assembly, Security Council, Economic and Social Council, Secretariat, and the International Court of Justice (ICJ).

General Assembly

UN General Assembly.

The General Assembly is the main deliberative organ of the United Nations. It is composed of all United Nations member states and meets in regular yearly sessions under a president elected from among the member states. At the start of each session all members have the opportunity to address the assembly over a two-week period. Traditionally, the Secretary-General makes the first statement, followed by the president of the assembly. The first session was convened on 10 January 1946 in the Westminster Central Hall in London and included representatives of 51 nations.
When the General Assembly votes on important questions, a two-thirds majority of those present and voting is required. Examples of important questions includes: recommendations on peace and security; election of members to organs; admission, suspension, and expulsion of members; and, budgetary matters. All other questions are decided by majority vote. Each member country has one vote. Apart from approval of budgetary matters, resolutions are not binding on the members. The Assembly may make recommendations on any matters within the scope of the UN, except matters of peace and security that are under Security Council consideration. The one state, one vote power structure theoretically allows states comprising just eight percent of the world population to pass a resolution by a two-thirds vote.

Security Council

Interior of the Security Council chambers.

Further information: Reform of the United Nations Security Council
The UN Security Council is charged with maintaining peace and security among countries. While other organs of the United Nations only make recommendations to member governments, the Security Council has the power to make decisions that member governments must carry out under the United Nations Charter. The decisions of the Council are known as United Nations Security Council Resolutions.
The Security Council is made up of 15 member states, consisting of five permanent seats and ten temporary seats. The permanent five are China, France, Russia, the United Kingdom and the United States. These members hold veto power over substantive but not procedural resolutions allowing a permanent member to block adoption but not to block the debate of a resolution unacceptable to it. The ten temporary seats are held for two-year terms with member states voted in by the UN General Assembly on a regional basis. The presidency of the Security Council is rotated alphabetically each month.
The Security Council has been criticised for being unable to act in a clear and decisive way when confronted with a crisis.

Economic and Social Council
The Economic and Social Council (ECOSOC) assists the General Assembly in promoting international economic and social cooperation and development. ECOSOC has 54 members, all of whom are elected by the General Assembly for a three-year term. The president is elected for a one-year term and chosen amongst the small or middle powers represented on ECOSOC. ECOSOC meets once a year in July for a four-week session. Since 1998, it has held another meeting each April with finance ministers heading key committees of the World Bank and the International Monetary Fund (IMF). Viewed separate from the specialized bodies it coordinates, ECOSOC's functions include information gathering, advising member nations, and making recommendations. In addition, ECOSOC is well-positioned to provide policy coherence and coordinate the overlapping functions of the UN’s subsidiary bodies and it is in these roles that it is most active.

Secretariat

The United Nations Secretariat is headed by the Secretary-General, assisted by a staff of international civil servants worldwide. It provides studies, information, and facilities needed by United Nations bodies for their meetings. It also carries out tasks as directed by the UN Security Council, the UN General Assembly, the UN Economic and Social Council, and other UN bodies. The United Nations Charter provides that the staff be chosen by application of the "highest standards of efficiency, competence, and integrity," with due regard for the importance of recruiting on a wide geographical basis.
The Charter provides that the staff shall not seek or receive instructions from any authority other than the UN. Each UN member country is enjoined to respect the international character of the Secretariat and not seek to influence its staff. The Secretary-General alone is responsible for staff selection.
The Secretary-General's duties include helping resolve international disputes, administering peacekeeping operations, organizing international conferences, gathering information on the implementation of Security Council decisions, and consulting with member governments regarding various initiatives. Key Secretariat offices in this area include the Office of the Coordinator of Humanitarian Affairs and the Department of Peacekeeping Operations. The Secretary-General may bring to the attention of the Security Council any matter that, in his or her opinion, may threaten international peace and security.

International Court of Justice

Peace Palace, seat of the ICJ. The Hague, Netherlands
The International Court of Justice (ICJ), located in The Hague, Netherlands, is the primary judicial organ of the United Nations. Established in 1945 by the United Nations Charter, the Court began work in 1946 as the successor to the Permanent Court of International Justice. The Statute of the International Court of Justice, similar to that of its predecessor, is the main constitutional document constituting and regulating the Court.
It is based in the Peace Palace in The Hague, Netherlands, sharing the building with the Hague Academy of International Law, a private centre for the study of international law. Several of the Court's current judges are either alumni or former faculty members of the Academy. Its purpose is to adjudicate disputes among states. The court has heard cases related to war crimes, illegal state interference and ethnic cleansing, among others, and continues to hear cases.
A related court, the International Criminal Court (ICC), began operating in 2002 through international discussions initiated by the General Assembly. It is the first permanent international court charged with trying those who commit the most serious crimes under international law, including war crimes and genocide. The ICC is functionally independent of the UN in terms of personnel and financing, but some meetings of the ICC governing body, the Assembly of States Parties to the Rome Statute, are held at the UN. There is a "relationship agreement" between the ICC and the UN that governs how the two institutions regard each other legally.

Secretary General

Ban Ki-moon, the UN's Secretary-General.
The Secretary-General of the United Nations is the head of the Secretariat, one of the principal organs of the United Nations, and acts as the de facto spokesman and leader of the United Nations.

Selection
The UN Charter provides little guidance for the selection of the Secretary General. The Charter states that "the Secretary-General shall be appointed by the General Assembly upon the recommendation of the Security Council". Over the years the process has changed, but always requires bartering and negotiation on the part of the five veto holding members of the Security Council.
In practice, a few details have remained consistent:
the Secretary-General is appointed for a renewable five year term
no Secretary-General has served more than two terms
candidates are selected using geographic rotation
no candidate has been elected from the country of a permanent member of the Security Council
the General Assembly has never rejected a candidate recommended by the Security Council

List of Secretaries General
Trygve Lie ( Norway) - February 1946 until his resignation in November 1952
Dag Hammarskjöld ( Sweden) - April 1953 until his death in a plane crash over Africa in September 1961
U Thant ( Burma) - November 1961 to December 1971
Kurt Waldheim ( Austria) - January 1972 to December 1981
Javier Pérez de Cuéllar ( Peru) - January 1982 to December 1991
Boutros Boutros-Ghali ( Egypt) - January 1992 to December 1996
Kofi Annan ( Ghana) - January 1997 to December 2006
Ban Ki-Moon ( South Korea) - January 2007 to present