At the most simple level, the term author refers to the writer of a book. However, in literary theory, things get more complicated. First, we don't actually encounter an author in reading as we would encounter a person in everyday life. Instead, from our reading, we infer an authorial voice or persona (the "implied author", "author function" or "narrative voice"); thus critics refer to the author as being "constructed" by readers.
Next, authorship is not monolithic. In oral traditional composition, legends, and many religious texts, the works we have are produced by a process of accretion and revision, often over thousands of years, and thus to speak of the Homeric epics or Prince Marko tales as having an "author" is at best problematic.
An author is a writer of a book, article, or report.