An embassy is a diplomatic mission of a country represented in another country, whereas a high commission is such a mission for a country that is a member of the Commonwealth of Nations. The heads of these missions are called ambassador and high commissioner, respectively.
The purpose of a diplomatic mission is to act as the official representation of the sending country in the host country. Other intergovernmental organizations, such as the United Nations, also maintain very similar diplomatic missions around the world. Countries, in turn, send representatives to these organizations. For example, every country in the world has a United Nations ambassador. In addition, smaller diplomatic missions called consulates are usually located in major centers. Their role is to aid with everything diplomatic, as well as travel and tourism between the home country and the host country.