Clojure comes with many useful functions that are a great learning resource for those who are just starting to explore the language.
Function doc gives us documentation on a given var, macro or special form. In an example below we see the documentation on the print function.

user=> (doc print)
-------------------------
clojure.core/print
([& more])
  Prints the object(s) to the output stream that is the current value
  of *out*.  print and println produce output for human consumption.
nil

Another very useful function is a dir macro. It prints out a sorted directory of public vars in a given namespace. The example below shows us how dir is used to get all functions from clojure.string namespace (a list was shortened for posting).

user=> (dir clojure.string)
blank?
capitalize
ends-with?
escape
includes?
...

Function apropos returns a sequence of all public definitions in all loaded namespaces that match the provided string or Regex pattern. For example if we run apropos with an argument “println” we will get all functions with names matching this string (with their namespaces).

user=> (apropos "println")
(clojure.core/println clojure.core/println-str)

We can access the source code of the built-in function definition directly from the Clojure Repl with the help of source macro. Reading the source code, especially when you are new to the language, helps greatly to grow as a software developer.

user=> (source print)
(defn print
  "Prints the object(s) to the output stream that is the current value
  of *out*.  print and println produce output for human consumption."
  {:added "1.0"
   :static true}
  [& more]
    (binding [*print-readably* nil]
      (apply pr more)))
nil

We can also expand the macro and see what it is doing “under the hood”. In the example below we use a macroexpand function on the defn macro. As we can see it actually uses a def special form to assign to a var sum an anonymous function.

user=> (macroexpand '(defn sum [a b] (+ a b)))
(def sum (clojure.core/fn ([a b] (+ a b))))