What is Compojure?
Compojure is a small, simple library that allows us to create specific request handlers for specific URLs and HTTP methods. In other words, "HTTP Method A requesting URL B will execute Clojure function C". By allowing us to do this, we can create our application in a sane way (URL-driven), and thus architect our code in some meaningful way.
Note
For the studious among us, the Compojure docs can be found at https://github.com/weavejester/compojure/wiki.