First, thanks to JJ for helping answer this question! He is exactly right. Because I'd like to link this post on the calendar by the antagonist exercise, let me quote JJ here:

"I suppose the best way to define the antagonist would be the direct source of conflict that the protagonist receives. For Example, in Harry Potter Voldemort would be the main source of conflict while if the story was told from Voldemort's perspective, Harry would be the antagonist." Very good point, JJ! It all depends on the perspective of the story.

Just in case you don't have one specific "bad guy" in your story, let me also explain that your ANTAGONIST does not have to be a person at all. Your antagonist is whatever causes conflict for your protagonist, which is your main character, with whom the reader empathizes (i.e., "roots for.")

So, for example: have you seen the TV Show, "My Name Is Earl"? In that show, if Earl does something bad, "Karma" is bad to him, and likewise, as he goes around making up for all the wrongs he's ever committed, Karma is good to him. Karma is not a person (although, for the purpose of the show, Karma is somewhat personified and given a personality). Karma is a supernatural force that drives what happens to Earl. When Karma is happy with Earl, he gets a date with a pretty woman. When Karma is angry with Earl, he gets hit in the face with a baseball. That sort of thing. If your antagonist is a personified non-character like this example, you might want to go ahead and write a character sketch. It will help you develop the personification and make it more believable.

However, it doesn't even have to be a "personified" non-character that causes conflict. It could just be karma or fate, with no personality. Something bad just happens, for no reason, and the protagonist has to deal with it. If this is the case, use your "Antagonist" exercise to describe the details. Did (s)he/it lose a loved one? Develop a disease? Get transfigured into a worm by an experiment gone awry, and now has to figure out how to change him/herself back without hands or a voice? Write about it.

And, I shouldn't forget to mention that the protagonist could also be your antagonist. Sometimes we're our own worst enemies. Did the protagonist make a bad decision that (s)he is now having to rectify? Write about it.

But if you DO have a main Bad Guy in your story, that character is definitely your antagonist. In that case, your antagonist exercise should be a traditional character sketch.

I hope this helps. If your novel project is away from the mainstream enough that I have not covered your scenario in my examples, feel free to send me an email with a quick premise, and I'll try to help you find your antagonist.