The protagonist of a work of fiction is the main character of that work. If the work has a "hero" the protagonist is quite probably the hero, but the term protagonist is broader than hero. The protagonist need not be a favorable character (one the audience is inclined to like or sympathize with), although s/he often is. The protagonist need not be the viewpoint character. For example, Sherlock Holmes is the protagonist of the stories about him, but the narrator and viewpoint character is his friend Dr. John Watson.
In some cases a work may not have a single clear-cut protagonist. Some works have many characters sharing the focus of interest, with no one of them obviously the protagonist (the classic "disaster movies" often follow this pattern, as do many of the longer works of Harry Turtledove).
Protagonists in spanking literature and film
In many spanking stories there are only two significant characters, a spanker and a spankee, and they often have a sufficiently equal focus that neither one really can be considered to be the protagonist. This is not always the case. In Lurking Dragon's "Melody stories", the character of Melody is obviously the protagonist.