The United Nations Security Council (UNSC) is one of the principal organs of the United Nations and its main responsibility under the Charter is to maintain international peace and security. Its powers, outlined in the United Nations Charter, include the establishment of peacekeeping operations, taking actions to prevent or stop aggression, and the authorization of military action. Its decisions are delivered in United Nations Security Council Resolutions.
The Security Council held its first session on January 17, 1946 at Church House, London. Since its first session, the Council has traveled worldwide holding sessions in many cities, such as Paris and Addis Ababa including its current permanent chamber in the United Nations building in New York City.

Contents:

1. Members

1.1 Permanent Members

1.2 Non-Permanent Members

2. The “Veto” Power and the Voting Procedure

3. The Status of the Non-members

4. Functions and Powers
 

1. Member States

There are 15 member states of the Security Council: five veto-wielding permanent members (China, France, the Russian Federation, the United Kingdom, the United States) and ten non-permanent members with two-year terms. This basic structure is set out in Chapter V of the UN Charter. Security Council members must be present at all times at UN Headquarters in New York to insure that the Security Council can meet anytime necessary.

1.1 Permanent Members

The Security Council's five permanent members are: China, France, the Russian Federation, the United Kingdom, the United States. In the beginning, in 1946, the Security Council consisted of France, the Republic of China, the United Kingdom, the United States and the USSR. There have been two changes to the permanent membership since 1946, although not reflected in Article 23 of the Charter of the United Nations as it has not been amended accordingly.

1.2 Non-permanent Members

Ten other members are elected by the General Assembly for two-year terms starting from January 1. Five new members are elected every year. The countries are elected based on the following criteria:
- Contribution to the maintenance of international peace and security
- Equitable geographic distribution (African countries have three members; the Latin America and the Caribbean, Asia, and Western Europe and Others have two members each; and Eastern Europe has one member. Also, one of these members is an Arab country, alternately from the Asian or the African continent.)

2. The “Veto” power and the Voting Procedure

Each member state has one representative at the Security Council. Each one of the states has one vote. Under the Charter the Security Council can establish its own rules of procedure and elect the President. Decisions on procedural matters i.e. the issues that may arise regarding the procedure are made by an affirmative vote of at least nine of the fifteen members. Decisions on all other matters, which are often referred to as substantive matters, require nine votes including the concurring votes of all five permanent members. This is the rule of "great power unanimity" commonly known as the "veto" power. A permanent member that does not agree with a decision can cast a negative vote and this has the power of veto. If a permanent member does not wish to support a decision but it also does not want to block it through a veto, it may abstain. All the permanent members have exercised the right of veto. The excessive use of the veto power was present during the Cold War which affected the functioning of the Security Council at the time. To overcome this difficulty that threatened to jeopardize the entire UN system, the General Assembly adopted the Resolution 337A known as the "Uniting for Peace" resolution in 1950. The Resolution states that if the Security Council fails to exercise its primary responsibility for the maintenance of international peace and security due to lack of unanimity of permanent members in any case where there appears to be a threat to the peace, breach of the peace, or act of aggression, the General Assembly can consider the matter and make recommendations for collective measures including the use of armed force when necessary.
Procedural matters are not subject to a veto. They are made by an affirmative vote of at least nine members.

3. The Status of the Non-members

A state that is a member of the UN, but not of the Security Council, if it is a party to a dispute under consideration by the Security Council may participate, without vote, in the discussion relating to the dispute.

4. Functions and Powers

Under the Charter the functions and powers of the Security Council are:
• to maintain international peace and security in accordance with the principles and purposes of the United Nations (Article 24, Chapter V of the Charter)
• to investigate any dispute or situation which might lead to international friction or give rise to a dispute (Article 34, Chapter VI of the Charter)
• to recommend methods of adjusting such disputes or the terms of settlement (Article 36, Chapter VI of the Charter)
• to formulate plans for the establishment of a system to regulate armaments (Article 41, Chapter VII of the Charter)
• to determine the existence of a threat to the peace or act of aggression and to recommend what action should be taken
• to call on member states to apply economic sanctions or other measures not involving the use of force to prevent or stop aggression
• to take military action against an aggressor (Article 42, Chapter VII of the Charter)
• to recommend the admission of new members and the terms on which States can become parties to the Statute of the International Court of Justice
• to exercise the trusteeship functions of the United Nations in strategic areas
• to recommend to the General Assembly the appointment of the Secretary-General and, together with the Assembly, to elect the Judges of the International Court.

In case a complaint concerning a threat to peace is brought before the Security Council its first action is usually to recommend parties to try to reach an agreement by peaceful means. The Council sometimes undertakes the investigation or/and mediation. It may also appoint special representatives or ask the Secretary-General to do so or to use his good offices. If a conflict leads to fighting, the Security Council usually issues cease-fire directives or deploys peace-keeping forces. Under Chapter VII it may also use other measures such as economic sanctions or collective military action. A state against which measures have been taken may be suspended from the exercise of the rights and privileges by the General Assembly on the recommended of the Security Council. If a state violates the rules of the Charter persistently it may be expelled from the United Nations by the General Assembly on the Security Council's recommendation.

Reference:
http://www.un.org/en/sc/
http://www.un.org/en/documents/charter/index.shtml
http://www.un.org/en/documents/charter/chapter5.shtml
http://www.un.org/en/documents/charter/chapter6.shtml
http://www.un.org/en/documents/charter/chapter7.shtml