The UN system
The International Monetary Fund, the World Bank and 12 other independent organizations known as specialized agencies are linked to the UN through cooperative agreements. These agencies, among them the World Health Organization and the International Civil Aviation Organization, are autonomous bodies created by intergovernmental agreement. They have wide-ranging international responsibilities in the economic, social, cultural, educational, health and related fields. Some of them, like the International Labour Organization and the Universal Postal Union, are older than the UN itself.
In addition, a number of UN offices, programmes and funds — such as the Office of the UN High Commissioner for Refugees (UNHCR), the UN Development Programme (UNDP) and the UN Children's Fund (UNICEF) — work to improve the economic and social condition of people around the world. They report to the General Assembly or the Economic and Social Council.
All these organizations have their own governing bodies, budgets and secretariats. Together with the United Nations, they are known as the UN family, or the UN system. Together, they provide technical assistance and other forms of practical help in virtually all economic and social areas.
(Source The UN in brief)