When U.S. diplomats negotiate a treaty, attend a state dinner, or arrange a visa for a traveler to the United States, they all have the same mission—to represent the interests and policies of the United States. Beyond that, diplomats’ roles and responsibilities are immensely varied.
An ambassador is the President’s highest-ranking representative to a specific nation or international organization abroad. An effective ambassador has to be a strong leader—a good manager, a resilient negotiator, and a respected representative of the United States. A key role of an ambassador is to coordinate the activities not only of the Foreign Service Officers and staff serving under him, but also representatives of other U.S. agencies in the country. At some overseas posts, personnel from as many as 27 federal agencies work in concert with embassy staff.
Foreign Service Officers are professional, trained diplomats who represent American interests abroad under the direction of the ambassador. All Foreign Service Officers listen to and observe what is going on in the host country, analyze it, and report to the ambassador and Washington. This makes U.S. policy more sensitive to the needs of other countries and their people. U.S. Diplomacy Center
Formal diplomacy is government-to-government diplomacy – also called Track I Diplomacy – that goes through formal, traditional channels of communication to communicate with foreign governments (written documents, meetings, summits, diplomatic visits, etc). This type of diplomacy is conducted by diplomats of one nation with diplomats and other officials of another nation or international organization.
Informal diplomacy includes Public Diplomacy which involves government-to-people diplomacy and reaching out to non-executive branch officials and the broader public, particularly opinion-shapers, in foreign countries, explaining both foreign policy and the national context out of which that policy arises. Public Diplomacy is carried out by both diplomats and, under their programs and auspices, non-officials such as academic scholars, journalists, experts in various fields, members of non-governmental organizations, public figures such as state and local government officials, and social activists. Source: U.S. Diplomacy Center