You've heard of these words before, the story's protagonist and the antagonist, and you probably even have an inkling of what they mean. So let's review.
The protagonist of the story is the main character, while the antagonist is the main character's opposite number, the one trying to settle the story's conflict in his or her favor, instead of our protagonist's. Notice that I didn't say hero and villain. While protagonist and antagonist have become deeply rooted in the way we discuss stories, we frequently forget that these words do not reflect the characters' moral fiber.
Rarely does a story have a clearcut single protagonist and single antagonist. It is more common to have a single protagonist, especially in a story written from the first person perspective or an extremely limited third person perspective.
For instance, in The Lord of the Rings, some would argue that Frodo is the lone protagonist, while others would say the entire Fellowship of the Ring (9ish characters) are the protagonists. In this case, there is a single, main antagonist - Sauron. However, many minor antagonists appear throughout the story, including a member of the Fellowship, Gollum, and the Wraith King.
Protagonistsand antagonists do not have represent good and evil. Instead, they represent the characters we relate to versus the ones we do not. Since the protagonist is the main character (although other characters can take the role of narrator), the reader will normally relate to them the most. In the case of multiple protagonists, the reader relates to them all. The best of example of this is the cast of Friends. A viewer can like one character more than another, but in the end, everyone is rooting for Ross and Rachel to get together.
The antagonist then, if not evil or the villain, is simply the character working against the protagonist of the story. Not every story has an antagonist character - sometimes the antagonist is the world, the setting, an outside influence, a disease, or even the protagonists themselves.
Next time you read a book, first ask yourself, who is my protagonist? And then use that protagonist to find your antagonist. The good thing about literature is that's open to interpretation - for example, is the protagonist of The Great Gatsby the narrator Nick Carroway or Gatsby himself?