Yields the unevaluated form. See http://clojure.org/special_forms for more information.
;; ' is the shortcut for quote
user> (= 'a (quote a))
true
;; quoting keeps something from being evaluated
user> (quote (println "foo"))
(println "foo")
=> *clojure-version*
{:major 1, :minor 5, :incremental 0, :qualifier "RC17"}
=> (quote)
nil
=> (quote 1)
1
=> (quote 1 2 3 4 5)
1
=> quote
CompilerException java.lang.RuntimeException: Unable to resolve symbol: quote in this context, compiling:(NO_SOURCE_PATH:1:42)
;; Proof that ' is just a syntactic sugar for quote:
user=> (macroexpand ''(1 2 3))
(quote (1 2 3))
;; Two ' must be used because reader processes and removes the first one