What is a protagonist?
A protagonist is usually the main character of a literary, or musical narrative, and they usually runs into problems caused by the antagonists. The audience is supposed to be able to figure out who the protagonist was. The terms protagonist and main character are explained and might indicate different things. In fiction, the story can be told from the perspective of a different character (could be the narrator). An example of this would be a narrator who relates the fate of protagonists. 
In Of Mice and Men
To see examples of protagonists in the book, click this.