An author is the creator of something. The person who writes a book is the author of that book. The person who creates an article for a magazine is also the author of the article. Someone who makes an essay is school is also the author of the essay. There are all kinds of authors, you can also define an author as the creator of something other than literature (writing) but generally most of the times when someone says the word "author" they usually mean someone who is responsible for writing something.