Here is the official definition of Model UN from the United Nations Association of the United States of America:
(http://www.unausa.org/)

"Model United Nations is an authentic simulation of the UN General Assembly, UN Security Council, or other multilateral body, which catapults students into the world of diplomacy and negotiation.
In Model UN, students step into the shoes of ambassadors of UN member states, from Afghanistan to Zimbabwe to debate current issues on the Organization's vast agenda.
The students, better known as "delegates" in Model UN, prepare draft resolutions, plot strategy, negotiate with supporters and adversaries, resolve conflicts, and navigate the UN's rules of procedure-all in the interest of mobilizing "international cooperation" to resolve problems that affect almost every country on Earth. Before playing out their ambassadorial roles in Model UN, students do research on the particular global problems to be addressed. The problems are drawn from today's headlines. Model UN "delegates" also look closely at the needs, aspirations, and foreign policy of the country they will "represent" at the event."


Having attended six MUN conferences myself, and been one of the principal organisers of one, I'd say that the above write-up is a better description of MUN as it exists at some levels.

However, it is important to realise what the aims are: to encourage students to think about the needs and views of people who may hold fundamentally opposed views to their own, and to make them argue for those people. When done properly, MUN can be one of the most eye-opening experiences available. Even when done badly, it is at the very least great great fun!