Technically this is the US Secretary of State; at this time that is Hillary Clinton (but is subject to change). It is her, or her office, that represents the United States to the rest of the world.
Obama
A developing nation.
It is a representative Republic.
the primary concern of a representative should be the interests of the nation (as a whole) or of his or her constituency
Representative Democracy
A democratic republic
Libran N. Cabactulan
A diplomat representing their country is referred to as an ambassador.
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A Delagate is A Representative to a convention or conference for example the United nation delagates with countries around the world
This question covers a broad area as "representing one's country" can mean as a government diplomat sent to another nation, or a president, who leads an entire nation and serves the nation as its top world representative. In lawmaking, or in creating treaties, elected officials in that line of work acts as a representative of one;s country. This can also carry on to athletics. For example in the Olympics, an athlete from Mexico becomes a representative of Mexico in the Olympic games.
The role that the president plays when he deals with foreign leaders is to negotiate on behalf of a nation. He acts as the representative of the entire nation.