The Six Principal Organs of the UN
(1) The General Assembly, which includes all of the 193 Member States.
(2) The Security Council with five permanent and 10 selected Member States.
(3) The Secretariat, headed by the UN Secretary-General.
(4) The Economic and Social Council, which includes 54 Member States.
(5) The International Court of Justice in The Hague.
(6) The Trusteeship Council.
In addition, the United Nations system includes a number of agencies, funds and programmes, for example, UNICEF (United Nations Children's Fund), UNDP (United Nations Development Programme), WFP (World Food Programme) and UNHCR (United Nations High Commissioner for Refugees).
You can read more about the UN system, here.