A cons, or dotted pair, is a compound data object having two components called the car and cdr. Each component may be any Lisp object. A list is a chain of conses linked by cdr fields; the chain is terminated by some atom (a non-cons object). An ordinary list is terminated by nil, the empty list (also written ()). A list whose cdr chain is terminated by some non-nil atom is called a dotted list.
The recommended predicate for testing for the end of a list is endp.