When you define a user option with defcustom, you must specify
its customization type. That is a Lisp object which describes (1)
which values are legitimate and (2) how to display the value in the
customization buffer for editing.
You specify the customization type in defcustom with the
:type keyword. The argument of :type is evaluated, but
only once when the defcustom is executed, so it isn’t useful
for the value to vary. Normally we use a quoted constant. For
example:
(defcustom diff-command "diff" "The command to use to run diff." :type '(string) :group 'diff)
In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Type Keywords).
Some type symbols do not use any arguments; those are called
simple types. For a simple type, if you do not use any
keyword-value pairs, you can omit the parentheses around the type
symbol. For example just string as a customization type is
equivalent to (string).
All customization types are implemented as widgets; see Introduction in The Emacs Widget Library, for details.