The UN's New York headquarters
The United Nations (UN), which emerged in 1945 from the devastation of global conflict, aims to "save succeeding generations from the scourge of war".
Its mission is to maintain international peace and security and to promote friendly relations between countries.
The UN Charter upholds human rights and proposes that states should work together to overcome social, economic, humanitarian and cultural challenges.
The UN's predecessor, the League of Nations, was established after the 1914-18 World War.
It aimed to prevent another global conflict, but it failed to halt the slide towards war in the 1930s and was disbanded in 1946.
Civil war in Liberia prompted the UN's biggest troop deployment
Much of the league's structure and many of its aims were adopted by its successor.
In 1944 the US, Britain, the Soviet Union and China met in Washington and agreed on a blueprint for a proposed world organisation.
The blueprint formed the basis of talks in 1945 between representatives from 50 countries. Under the terms of the resulting charter the UN came into being on 24 October 1945.
The UN comprises 193 member states. South Sudan is the newest member - it joined in July 2011. Membership grew as colonies became independent and the Soviet Union disintegrated. The Vatican and Taiwan remain non-members. Most members have permanent missions at the UN's main headquarters in New York.
Potential members are recommended by the Security Council and are admitted by a two-thirds majority vote in the General Assembly.
Member nations contribute to the running costs of the UN. A country's contribution is assessed on its ability to pay. The US is the top contributor.
The assembly is the UN's main forum for debate. It is the only UN body which includes representatives from all member countries. Each member country has one vote.
Members can discuss any subject in the UN Charter, from international security to the UN budget. The assembly can issue recommendations, based on its deliberations. But it has no power to force countries to act on these.
The assembly may also adopt "declarations", reflecting high degrees of concern or resolve among members.
On key issues - including international security - a two-thirds majority is needed to adopt a resolution.
The General Assembly meets for three months of the year from mid-September, and for special and emergency sessions. Its annual sessions open with a "General Debate", in which each member country delivers a statement about its perspective on world events.
Most assembly business is dealt with by its six Main Committees. The assembly approves or rejects their recommendations.
The council is tasked with ensuring global peace and security. It has five permanent member nations: China, France, Russia, the UK and the US.
Ten other countries have temporary membership on a rotating basis.
The council can impose economic sanctions and can authorise the use of force in conflicts.
It also oversees peacekeeping operations.
Economic and Social Council
The council spearheads the UN's economic, social, humanitarian and cultural activities.
It oversees the work of commissions which deal with human rights, population growth, technology and drugs, among other issues.
Its 54 members are elected by the General Assembly.
International Court of Justice (World Court)
The court is the main judicial body of the UN and is tasked with settling legal disputes submitted to it by states. It sits in the Dutch city of The Hague.
SOME RECENT COURT RULINGS
2011: Rules that it does not have jurisdiction to examine case filed by Georgia that accuses Russia and separatist rebels of ethnic cleansing.
2010 Rules that Kosovo's declaration of independence from Serbia in 2008 did not break international law.
2006: Rules against Argentina's attempt to have paper mill projects in Uruguay suspended
2005: Rules that Uganda must compensate DR Congo for looting during 1998-2003 war
2004: Majority ruling that parts of Israel's West Bank barrier built on Palestinian land are illegal
2004: Orders review of convictions of 51 Mexicans on death row in US
2002: Clears way for Bosnia to seek compensation from Belgrade over 1992-95 war
2002: Awards sovereignty of Bakassi peninsula, claimed by Cameroon and Nigeria, to Cameroon
The court's 15 judges are elected by the General Assembly and the Security Council.
The court's decisions are binding, although nations have sometimes refused to accept its rulings.
The Secretariat undertakes the day-to-day work of the UN, administering the programmes and policies of the organisation. Its work includes research, translation and media relations. Some 9,000 Secretariat staff are drawn from 170 countries.
The council administered the UN's trust territories. It suspended its activities in 1994 when the last of the trust territories, Palau in the south Pacific, became independent.
The council, made up of the five permanent Security Council members, agreed in 1994 to meet "as occasion required".
THE UN SYSTEM
Fourteen independent agencies make up the "UN System" alongside many of the organisation's own programmes and agencies.
The independent agencies include the World Bank, the International Monetary Fund and the World Health Organisation. They are linked to the UN by cooperation agreements.
Fighting hunger: UN's World Food Programme at work in Afghanistan
The UN's own major agencies and programmes include:
- International Atomic Energy Agency (IAEA) - UN's nuclear watchdog, based in Vienna
- International Criminal Tribunal for the Former Yugoslavia (ICTY) - set up to try those suspected of war crimes in the former Yugoslavia, based in The Hague
- UN Children's Fund (Unicef) - promotes child health, education, protection
- UN Development Programme (UNDP) - committed to reducing poverty, developing infrastructure, promoting democratic governance
- UN Educational, Scientific and Cultural Organisation (Unesco) - based in Paris, aims to promote peace and development through education, science, culture and communication
- UN Environment Programme (Unep) - based in Nairobi, promotes environmental protection
- UN High Commissioner for Refugees (UNHCR) - safeguards rights and well-being of refugees; based in Geneva
- UN Works and Relief Agency (UNWRA) - dedicated agency providing assistance solely to Palestinian refugees and their descendants
- World Food Programme - based in Rome; "frontline agency in the fight against hunger"
Secretary-general: Ban Ki-moon
Ban Ki-moon describes himself as an expert mediator
1946-1952 - Trygve Lie (Norway)
1953-1961 - Dag Hammarskjöld (Sweden)
1961-1971 - U Thant (Burma)
1972-1981 - Kurt Waldheim (Austria)
1982-1991 - Javier Perez de Cuellar (Peru)
1992-1996 - Boutros Boutros-Ghali (Egypt)
1997-2006 - Kofi Annan (Ghana)
Ban Ki-moon, a former South Korean foreign minister, took up the post on 1 January 2007.
He was re-elected for a second term by the UN General Assembly, unopposed and unanimously, in June 2011, with effect from 1 January 2012.
Mr Ban, who is the first Asian secretary-general for 35 years, describes his priorities as mobilising world leaders to deal with climate change, economic upheaval, pandemics and increasing pressures involving food, energy and water.
In style, he prefers quiet diplomacy and sees himself as a bridge-builder, aiming to give voice to the world's poorest and most vulnerable people, and to strengthen the UN itself, which was dented when he took office by scandals over the UN oil-for-food programme in Iraq, and corrupt procurement.
He was born in Chungju, Korea, in 1944 and studied international relations at Seoul University. He worked at South Korea's UN mission before joining the government.
He succeeded Kofi Annan, a Ghanaian, who became the UN's seventh secretary-general in 1997. He was re-elected for a second term in 2001.
The General Assembly elects the secretary-general for a five-year renewable term. The post is often filled by candidates from smaller, neutral nations.
The US-led war in Iraq in 2003 - launched without Security Council authorisation - led to apocalyptic predictions of the collapse of the council and of the UN system.
Deadly attack on its Baghdad HQ forced UN to rethink its role in Iraq
Secretary-General Kofi Annan warned that pre-emptive attacks "could set precedents that result in a proliferation of the unilateral and lawless use of force".
He added that the UN could only head off unilateral military responses by showing that it could respond collectively to the security concerns of individual countries.
The possibility of pre-emptive action would mark a major departure from the charter, which advocates containing threats through containment and deterrence.
The UN has taken on an increasingly interventionist approach since the end of the Cold War, the tense stand-off between the Soviet bloc and the West which dominated much of the organisation's first four decades.
But despite some successes in the peacekeeping arena, operations in Bosnia, Rwanda and Somalia were flawed, failing to prevent massacres and even genocide.
A 2000 report criticised the UN's insistence on neutrality in situations where one side resorted to violence, warning that this could render missions ineffective.
The organisation's blue and white olive branch motif does not guarantee safety; more than 1,500 peacekeepers have been killed since the UN's inception.
The share-out of power in the UN, particularly in the Security Council, is hotly debated. Critics say the over-riding influence of the council's five permanent members is unfair.
The former secretary-general Kofi Annan, among others, has described the structure of the council as anachronistic. The major powers on the Security Council oppose moves to give more power to the General Assembly.
The UN came under heavy fire in 2005 when an investigation into the oil-for-food programme that it operated with Saddam Hussein's Iraq found that the scheme had been mismanaged and was riddled with corruption. It criticised the secretary-general for failing to oversee the programme adequately.
Peacekeeping operations have been hit by accusations of fraud, as well as charges of sexual abuse by peacekeeping troops.
Having been mired in a financial crisis for many years, the UN has come under pressure to cut spending and to slim down its bureacracy. Member nations owe the organisation billions of dollars.