Clojure : loading dependencies at the REPL

I recently learned (thanks to technomancy) that, at the REPL ---

This fails:

user=> (:require [clojure.set :as set])
java.lang.ClassNotFoundException: clojure.set (NO_SOURCE_FILE:24)

Whereas this succeeds :

user=> (require '[clojure.set :as cs]) 
nil

at loading the clojure.set class.

Context: The former line was copied from a namespaced source file.

My primary question is : What is the change we have made, by swapping the : and ' characters, which now allows for success of the latter command ?

My 2nd question is , in general - what are the guidelines for doing things at the REPL --- as compared with doing things in normal clojure source files ? Assume here that we can load our repl from the root of a LEININGEN project, so at least the jars will be available on disk in the dependencies sub directory.