append makes a new list consisting of the members of its argument lists. append takes any number of arguments.
(append '(a) '(b)) = (a b)(append '(a b) '(c d)) = (a b c d)
(append '(a) '(b) '(c)) = (a b c)
(append '((a b)) '((c d))) = ((a b) (c d))
reverse makes a new list that is the reverse of the top level of the list given as its argument.
(reverse '(a b)) = (b a)(reverse '((a b) (c d))) = ((c d) (a b))
length returns the length of the top level of the list given as its argument.
(length '(a)) = 1 (length '(a b)) = 2 (length '((a b))) = 1