LISTP function returns true if the argument is refers to object of type list; otherwise it returns false. Objects of list type can contain cons cells or NIL value (list terminator). See CONS, CONSP and LIST.

(listp nil) => T
(listp 'some-symbol) => NIL
(listp 3) => NIL
(listp "moo") => NIL
(listp (cons 1 2)) => T
(listp '(1 . 2)) => T
(listp '(1 2 3 4)) => T
(listp (list 1 2 3 4)) => T