What is the United Nations?

In Social Studies we were talking about how the United States joined the United Nations in 1945.  Some questions then arose.  This is where we need our Super Smart Students to do some researach and help us out.  If someone answers one of the questions, please don't repeat the same answer.  You may do further research and share a fun fact to further expand our knowledge though.  Here are your United Nations Questions:
-What is the United Nations?
-When was the United Nations established?
-How many countries make up the United Nations?
-What are people called that serve in the United Nations?
-Who is the representative for the United?
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