A foreign minister is a cabinet minister that helps to form foreign policy for sovereign nations. The Ministry of Foreign (or External) Affairs is often considered to be the most senior ministerial position after Prime Minister, and is often given to the Deputy Prime Minister in coalition governments.
A Foreign Minister's powers can vary from government to government. In a classic Parliamentary system a Foreign Minister can have genuine influence on forming foreign policy. However, with a strong Prime Minister or in a Presidential system, a Foreign Minister can be more advisory or outright ignored. Since the end of World War II, it has been common for the foreign minister to be part of an inner cabinet, often known as a National Security Council with the defense minister so that the defense and diplomatic policy can better be coordinated.
While it was common for a head of government to assume the foreign ministry in the 19th or early 20th centuries, this is not as common anymore. It does still happen though, however it is mostly confined to newly independent or still developing nations.
In the United States, the Foreign Minister is called the Secretary of State, and is the oldest cabinet post in the US. In the United Kingdom, the Foreign Minister is called the Foreign Secretary, and has officially been called the Secretary of State for Foreign and Commonwealth Affairs since 1968, when the previously separate Foreign Office and Commonwealth Office were merged.