"Membership of the United Nations is open to all peace loving states..."

So begins article 4 of the idealistic UN charter. The number of member states has grown from 51 (in 1945) to 189 more than half a century later.

The United Nations is made up of a General Assembly and a Security Council (among a lot of other standing committees and organizations --bookw56). The former includes a representative from every member state who put forward and debate various resolutions (the most famous being the Universal Declaration of Human Rights). The latter, more powerful body arbitrates disputes, authorises the imposition of economic sanctions or even recommends the intervention of UN peacekeeping troops in world flash points.

Current members of the Security Council with the date at which their membership expires (permanent members in bold)

Current UN peacekeeping missions are being conducted in 15 locations around the world. Each mission is given a name, usually UNxxx. The pertinant nations and mission names are-

The member nations of the United Nations are listed here.