NIL symbol denotes empty list and false value. There is also ubiquitous NIL constant which contains NIL symbol. NIL is considered as false value by comparison functions and control operators (unlike any other). Empty list, that is '() or even (), is the same as NIL value.
nil => NIL 'nil => NIL () => NIL '() => NIL
(eq nil 'nil) => T (eq 'nil ()) => T (eq () '()) => T
(not t) => NIL (not nil) => T (not 234.3) => NIL