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.
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.
Profile: UN Security Council
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.