America (Ameriko) is the name of the continent, while the USA (Usono) is just one country of it. It is great that esperanto made this distinction because it seems that the citizens of the USA (usonanoj) dont have it clear. Both North and South Americans are Americans (amerikanoj)
Well... sort of. In most of the English-speaking world, "American" is the adjective meaning "related to the USA", and "America", in most contexts, means "The USA."
To Georgie's question - yes, in a properly contrived context it could mean that.