The Secretariat is actually one of the main organs of the United Nations.  Unlike the other organs, however, which are comprised of States represented by their Governments, the Secretariat is staffed by an international civil service that works under the direction of the Secretary General.

The Secretariat carries out the day-to-day work of the Organization.  It services the General Assembly, Security Council, ECOSOC, International Court and Trusteeship Council and carries out tasks as varied as the issues dealt with by the UN: administering peacekeeping operations, surveying economic and social trends, preparing studies on human rights, among others.

In addition to its offices at UN Headquarters in New York, the United Nations also has offices in Geneva, Nairobi and Vienna and Regional Economic Commissions in five regions:

United Nations Office at Geneva (UNOG)
United Nations Office at Nairobi (UNON)
United Nations Office at Vienna (UNOV)
Economic Commission for Africa (ECA), Addis Ababa, Ethiopia
Economic Commission for Europe (ECE), in Geneva, Switzerland
Economic Commission for Latin America and the Caribbean (ECLAC), in Santiago, Chile
Economic and Social Commission for Asia and the Pacific (ESCAP), in Bangkok, Thailand
Economic and Social Commission for Western Asia (ESCWA), in Beirut, Lebanon

The United Nations also staffs and operates several programs, funds and research and training institutes.  Examples include:

Programmes and Funds

Office of the United Nations High Commissioner for Refugees (UNHCR)
Office of the United Nations High Commissioner for Human Rights  (OHCHR)
United Nations Children’s Fund (UNICEF)
United Nations Conference on Trade and Development (UNCTAD)
United Nations Development Programme (UNDP)
United Nations Volunteers (UNV)
United Nations Drug Control Programme (UNDCP)
United Nations Environment Programme (UNEP)
United Nations Human Settlements Programme (UN-HABITAT)
United Nations Population Fund (UNFPA)
UN Entity for Gender Equality (UN Women)
United Nations World Food Programme (WFP)

Research and Training Institutes

United Nations Institute for Disarmament Research (UNIDIR)
United Nations Institute for Training and Research (UNITAR)
United Nations Interregional Crime and Justice Research Institute (UNICRI)
United Nations Research Institute for Social Development (UNRISD)

Other UN Entities

United Nations University (UNU)