I recently started learning Clojure and I'm having a bit of difficulty wrapping my head around namespaces. As the creator of Clojure said, newcomers often struggle to get the concept right. I don't clearly understand the difference between (use ...) and (require ...). For example playing around in the REPL if I say (use 'clojure.contrib.str-utils2) I get warnings about functions in clojure.core namespace being replaced by the ones in clojure.contrib.str-utils2, but that doesn't happen when I use (require 'clojure.contrib.str-utils2). I'm not sure that I will always want to replace what's in clojure.core, so can someone point some best practices for importing external stuff and managing namespaces in Clojure?
Oh and also, when should I use :use and :require? Only inside (ns ....)?
Thanks in advance.