An antagonist is the character working against the protagonist. Sometimes they are considered a villain, but not always. It's the character that is opposite of your antagonist and is standing in the way of the hero reaching their goal. Here's a small, important twist. While an antagonist is usually thought of as a person, sometimes it can be something else. Take a story like Cast Away with Tom Hanks. There is not another person, but he is isolated on an island and must deal with nature. In this instance, nature (think weather, survival) is the antagonist. It creates conflict for the character.