The IOC (International Olympic Committee) is the main Olympic organiser. they are the group at the head of the Olympics. There website is - http://www.olympic.org/uk/index_uk.asp
The NOC (National Olympics Committee) is the subsection of the international one. it is based in each country eligible to participate. The Irish NOC website is - http://www.olympicsport.ie/Content.aspx?pageid=60
Over 200 countries attended and participated in the Sydney 2002 Olympics even the there is only 192 countries in the world. How is this possible?? This is because the IOC (International Olympic Committee) also recognises independent territories, commonwealths, protectorates and geographical areas. Of the total of 192 countries only three do not have a NOC (National Olympic Committee). These are the Kiribati, Marshall Islands and The Vatican City. That means these countries were not permitted to take party in the Sydney Olympics. This leaves 189 with NOCs. When you add the 12 territories, protectorates and commonwealths (American Samoa (U.S.), Aruba (Netherlands), Bermuda (U.K.), British Virgin Islands (U.K.), Cayman Islands (U.K.), Cook Islands (New Zealand), Guam (U.S.), Hong Kong (China), Netherlands Antilles (Netherlands), Palestine, Porto Rico (U.S.), and the Virgin Islands (U.S.)) who have NOCs to that and can then participate in The Games you get 201 eligible to participate. Currently Afghanistan is not eligible to participated in the Olympics due to Lack of Control Afghanistan's NOC has on what athletes are sent to the Olympics. This is because the IOC believes that the Taliban are in control of those things.