These countries were all allies in World War II, which they won. They are also all nuclear weapons states. A total of 15 UN member states serve on the UNSC, the remainder of which are elected. Only the five permanent members have the power of veto, which enables them to prevent the adoption of any "substantive" draft Council resolution, regardless of its level of international support.
At the UN's founding in 1945, the five permanent members of the Security Council were the French Republic, the Republic of China, the Soviet Union, the United Kingdom, and the United States. There have been two seat changes since then, although not reflected in Article 23 of the United Nations Charter as it has not been accordingly amended:
Additionally, France reformed its provisional government into the French Fourth Republic in 1946 and later into the French Fifth Republic in 1958, both under the leadership of Charles de Gaulle. France maintained its seat as there was no change in its international status or recognition, although many of its overseas possessions eventually became independent.
The five permanent members of the Security Council were the victorious powers in World War II and have maintained the world's most powerful military forces ever since. They annually top the list of countries with the highest military expenditures; in 2011, they spent over US$1 trillion combined on defense, accounting for over 60% of global military expenditures (the U.S. alone accounting for over 40%). They are also five of the world's six largest arms exporters, along with Germany and are the only nations officially recognized as "nuclear-weapon states" under the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), though there are other states known or believed to be in possession of nuclear weapons.
The "power of veto" refers to the veto power wielded solely by the permanent members, enabling them to prevent the adoption of any "substantive" draft Council resolution, regardless of the level of international support for the draft. The veto does not apply to procedural votes, which is significant in that the Security Council's permanent membership can vote against a "procedural" draft resolution, without necessarily blocking its adoption by the Council.