Clojure Libs and Namespaces: require, use, import, and ns
Good overview of how namespaces work in Clojure 1.2. Most people just need to learn 'ns' to get started, but if you want to get into some of the details, read this post:
http://blog.8thlight.com/articles/2010/12/6/clojure-libs-and-namespaces-require-use-import-and-ns
