the U.N.

     The U.N. means the United Nations. The United Nations is a sort of  "congress of the nations" that meets in New York, in the United Nations building.  The members of the United Nations are countries.  Each member country sends a representative or ambassador to the General Assembly, where countries can meet and talk about their shared goals and their differences.  You may be most familiar with news reports of the United Nations  peacekeeping forces--something like the army of the U.N. Each member state ( country or nation ) is supposed to contribute troops and supplies when needed, according to the size or population of the state.
    The United Nations also had a court, called the World Court, that meets in the Netherlands, often called Holland. You may have seen the news reports of the trials of some of the military leaders of Eastern Europe, after the war in what was then called Yugoslavia. Most of the disputes heard in the court have to do with borders or property, but some criminal cases are also brought to the World Court.
     The United Nations is headed by a Secretary general, elected by the members of the General Assembly. To describe what it does on a daily basis would be difficult. Dozens of international organizations are actually part of the United Nations--UNICEF ( for children ), WHO ( the World Health Organization ), UNESCO (  a cultural organization which may protect historic sites ), and many others. Each member state pays "dues" to the United Nations, part of which are used to sponsor relief , health, and educational organizations.