An ambassador is the highest ranking diplomat who represents a nation and is usually accredited to a foreign sovereign or government, or to an international organization.
Sometimes countries also appoint highly respected individuals as Ambassador at Large who are assigned specific responsibilities, and they work to advise and assist their governments in a given area. The word is also often used more liberally for persons who are known, without national appointment, to represent certain professions, activities and fields of endeavor.
In everyday usage it usually applies to the ranking government representative stationed in a foreign capital. The host country typically allows the ambassador control of specific territory called an embassy, whose territory, staff, and even vehicles are generally afforded diplomatic immunity in the host country.