The ancestor of the UN is the concept of the Concert of Europe, formulated in the 19th century after the Napoleonic wars. The idea was that the Great Powers would work together by meeting together. The peace of Europe was largely maintained from 1814 until 1870 - at least between France, Britain and Germany.
Following the first world war President Woodrow Wilson proposed a League of Nations to extend the principle of Concert (working together) to the whole planet. However, the League failed to deal with the rise of the extreme nationalist states: Germany, Italy and Japan, and failed to prevent the wars associated with their rise.
Following the second world war, in which the allies called themselves the "United Nations", it was agreed to form a world organization to replace the League of Nations.
The statesmen of 1944, F.D.Roosevelt and Winston Churchill, said they wanted a forum to discuss world problems and provide collective military and other response to threatened wars. They set up a two tier structure for world meetings: The General Assembly in which every state, however small, had one vote; the Security Council which was limited to the Great Powers and representatives of the other powers by rotation.
The Permanent Members of the Security Council had a Veto, the power to prevent formal agreement. This represents a realistic desire to avoid empty resolutions and a recognition of the fact that the large powers cannot be compelled to act against their wishes.
Permanent members in 1945 were: United States, Soviet Union, Britain, France, China.
If they were supposed to be the undisputed World Powers this list is due for revision, as Germany and Japan might qualify while Britain and France no longer have world power, after losing their colonies. A suggestion has been made of giving one seat to Japan and the other to the European Union, which might allocate it to the rotating European presidency. The United States has proposed that Germany and Japan be given Permanent status, but if Britain and France retain their seats there will be strong demands from such countries as India and Indonesia for permanent status too. The Security Council might then become too large to make effective decisions. These changes are being reflected in the G20, a less formal grouping in chich India and China are represented, along with other powers such as Brazil.
Following the end of the Cold War collective action has become more frequent.
UN forces are now located in Kuwait, Iraq, Cyprus, Somalia, the Balkans, Lebanon, Congo, Bosnia. The Gulf War of 1991 was in name a UN war.
The administration consists of:
Secretariat at the New York HQ to run the General Assembly and Security Council
Economic Commissions for Africa, Europe, Latin America and Caribbean, Asia and Pacific, Western Asia.
UN Center for Human Settlements
UNCTAD Conference on Trade and Development
UNEP Environment Program
World Food Council
many were taken over from the League of Nations
FAO Food and Agriculture Organization
IAEA International Atomic Energy Agency
(Is it poacher or gamekeeper?)
ICAO International Civil Aviation Org.
ILO International Labor Org.
IMO International Maritime Org.
UNESCO UN Educational, Scientific & Cultural Organisation.
(US and UK boycotted it for a number of years because of alleged bias against freedom of the press)
UPU Universal! Postal Union (dates back to before the League of Nations)
WHO World Health Org. One of the most effective (elimination of smallpox)
WIPO World Intellectual Property Org. (copyright)
WMO World Meteorological Org.
UNDP UN Development Program
UNFPA Fund for Population Activities
UNHCR UN High Commission for Refugees
UNICEF UN Children's fund (originally International Children's Emergency Fund, but the emergency has lasted for 48 years already)
UNIDO UN Industrial Development Org.
UNITAR UN Institute for Training & Research
UNRWA UN Relief and Works Agency (Palestinian refugees)
WFP World Food Program
Interpol Police cooperation
World Bank (International Bank for Reconstruction and Development
IMF International Monetary Fund
(The money is controlled by those with most of it, the western powers, and voting is strictly ad valorem, as in a joint stock corporation)
Switzerland has voted to join (March 2002)
Taiwan (officially part of China)
Vatican (The HQ of a rival world body)
Does the UN represent the world as a whole? In the Kuwait war the military action was taken as a result of UN resolutions. However, these were passed at the request of the United States and Britain. Did the rest of the world do more than assent?
Some critics observe that the Security Council and General Assembly have both passed resolutions on Israel and Palestine. No collective action, other than an observer force, has resulted. The assumption is that the Great Powers will act only when their own interests are affected (such as oil supplies in Kuwait). This loses the support of all Muslim powers who tend to see the UN as something got up by the former colonial powers, still controlling it.
The funds are provided according to National Income. Thus the US is supposed to pay 25%, Japan 12.5%, Russia 9.4%. There is no power on earth to prevent the payers from calling the tune. In addition the US (as well as some others) is in arrears with its payments which keeps the UN short of funds. Of course there is a reason for this: Uncle Sam is not short of the readies. One proposal is an independent source of funds such as a small tax on currency transactions, or rent on the seabed.
Therefore when the General Assembly passes a resolution nothing will happen if a majority only of low payers vote for it. During the Cold War the US and the USSR could withhold aid if the small powers voted the wrong way. As recently as 1990 Yemen lost its US aid when it voted the wrong way in the Security Council over Iraq's invasion of Kuwait. There are said to be similar pressures in 2003 on the smaller, non-permanent members of the Security Council over the proposed war with Iraq.
Probably only the SF writers' invasion from outer space could turn the UN into a planetary government.
Actual world policy decisions seem to be being made these days by the more informal meetings of the G20 group of heads of government (formerly the G7).