At first sight MUN may look very similar to the UN, but with careful observation, one may understand the differences. The most significant difference is that MUN is totally run by students and that not a single adult/teacher is responsible for anything.
Starting from the President and the Secretary General, every position that exists in the UN is represented by students. Students that hold staff positions are usually chosen from the schools that are near to the MUN location since they are the ones that will have to prepare for it in advance. Many committee chairs and deputy chairman are chosen from a group of students around the world that apply for these positions.
Further more, the United Nations delegates are actively involved in the world's problems for years. The UN is also largely based on nationalism, as delegates represent their own country and deal with problems in the real world. (Which would directly concern their rights and well-being.) In the MUN it is different because every student represents another country, thus it is not based on nationalism. What is interesting is that because each delegation represents another country, they may tend to support their own country when the country they represent doesn't have anything to do with them.
The MUN has no acting power so, it cannot put to action any of the resolutions that pass through, however, The Hague Model United Nations has been recognised by the UN as a Non-governmental Organisation (NGO). Because of this, THIMUN resolutions will be sent to the UN, and official UN delegates will take them into consideration.
MUN is often considered somewhat more effective than the United Nations in achieving global harmony. For example, some believe that when the UN comes to a decision, it has to be waved through by the permanent five members of the Security Council. (United States, United Kingdom, France, China and Russia) So in other words, these five countries have a distinct influence on all issues of the UN, which in turn, leads to corruption and one-sidedness. With this, the foundation by which the UN was established, is slowly chipped away. Instead of trying to reach solutions to the worlds problems, the UN becomes a stage for each country pursuing their national interests, and in the end the rich countries will prevail. MUN, on the other hand, seems to aviod this particular weakness.
Similarities also exist between these two organizations. Every part of the UN is included in MUN. All the committees special meetings have been included. One of the only MUN conferences that includes all the countries that the UN has is The Hague International MUN. Other similarities include the formation with a General Assembly, in which every delegation is present, a Security Council, a group of 15 countries, 5 of which are permanent, and various Commissions that exist in both. Along with the structure, the goal of reaching solutions to make this world a better place is very similar, if not completely identical.