An antagonist is a character that provides opposition to the protagonist, or main character, in a story. This character or group of characters creates conflict for the main character.
An antagonist typically contrasts the protagonist's qualities and provides conflict for him. The antagonist is critical for a story's plot development, climax and story resolution because of his contradiction to the main character. In some cases, the antagonist can also be the protagonist if the character causes his own moral or inner conflict. Some examples of antagonists in a story are President Snow to Katniss in the "Hunger Games" series, the Joker in "Batman" and Darth Vader to Luke Skywalker in "Star Wars."