I am American. Elijah is correct in saying that the Americas are the two continents, North and South America. The three major countries of North America are Canada, Mexico, and the United States of America. Citizens of Canada are Canadians. Citizens of Mexico are Mexicans. But what do you call citizens of a country with the difficult name of "the United States of America"? The only thing that makes sense is "American". So citizens of the United States are Americans. If someone asks me where I am from, I generally say "I am from the United States" to avoid confusion. If someone asks me for my nationality, I say "I am American". USA is simply the abbreviation for United States of America. It is generally used writing as an abbreviation for the country. It is rarely used in speech. In speech it is more common to use "the United States". Example: The United States just elected Donald Trump. Sometimes you will hear (or read) the US rather than "United States". Example: The US just elected Donald Trump. Notice that it is almost always "the United States" and not "United States".

There's no difference. The USA in abbreviated form stands for the "The United States of America" so someone could say 'I am from the United States of America' or 'I am from America' and there is no difference in the meaning. I hope that I provided an adequate answer to your question.

America is the continent, USA means United States of America and is a country, many people from USA say they are Americans when they speak about the country, sometimes is confusing because people Mexico, Canada, and many other countries are also in America so they are also Americans.

Karl gave a good answer. I think that the word "American" is a more spoken, colloquial way of saying you're either from the US or, more specifically, a US citizen. If someone says "he's American" I automatically think he's a US citizen, and certainly not Mexican or Canadian. Now if you say, I come from the US (or USA), that's doesn't necessarily imply citizenship. So that's kind of the big difference to me. You could just be a resident alien and still say you are from the US. That just tells someone you live there. And like Karl said, don't forget the "the" when using US or USA.

'America' is a continent and U.S.A. is a Country belonging to that continent, but in Italy when we say 'America' we almost always mean 'U.S.A.', as a synecdoche. We also use a synecdoche to refer to the United Kingdom: we say 'Inghilterra' (England) to mean 'U.K.'. But as you can see, the latter is an 'inverted synecdoche' compared to the former: as to the U.S.A. we say the 'bigger' (a continent) to mean the 'smaller' (a Country), while as to the U.K. we say the 'smaller' (part of the Union) to mean the 'bigger' (the whole Union).