The UN (aka The United Nations) is an international organization that describes itself as a "global association of governments facilitating cooperation in international law, international security, economic development, and social equity."
It was founded at the end of World War II by the 'victorious world powers', who hoped that it would act to prevent conflicts between nations by providing a forum in which all countries could address their concerns nonviolently.
Its headquarters are in New York City, and it is devided into administrative bodies such as the UN General Assembly, UN Security Council, UN Economic and Social Council, UN Trusteeship Council, UN Secretariat, and the International Court of Justice. It also includes WHO and UNICEF.
There is, obviously, a lot of debate over how well the UN is doing its job. Of course this is just debate and the whole matter is sensitive.