The protagonist is the chief character in a story. The term comes from the Greek meaning "chief actor." Ancient Greek drama began with all action and dialogue being delivered by the chorus. Aristotle noted how the playwright Thespis introduced the idea of having a single actor step forward and engage in conversation with the chorus. This was the creation of the protagonist.
The protagonist is the central figure in a novel, play, short story, or film. He or she is the one driving the plot forward. Often the protagonist is the character with which the audience is most supposed to identify. The protagonist is usually, but not always, identified as the hero by the audience. However, numerous modern stories set up their protagonist as an anti-hero, defying convention and moving the drama forward through this disconnect. The character of Walter White from the TV show Breaking Bad, for example, is clearly the protagonist of the entire series. However, Walter White's actions are anything but heroic, as he engages in lying, murder and cooking meth in order to amass a fortune.
Still, the protagonist is the emotional center of the story. An audience in compelled to follow the twist and turns of Walter White's story in the same way that we follow Hamlet through his dilemma, or Batman's quests to rid Gotham City of villains. Even in the midst of a large ensemble, the protagonist's actions are central to plot.