What is Expository Composition

What is Expository Composition? I think it has something to do with writing. Compostition has a meaning that has to do with writing, so exposing your writing maybe? But Dr. Preston is the teacher, so Open Source Learning is involved in this class obviously. Expository Composition is also maybe writing about literature? Maybe old literature, since the name is kind of formal? I really have no clue, i can make some educated guesses however...