The 'and' special form evaluates a sequence of expressions and returns
the effect of a logical AND on the expressions. 
(and t t t) => T (and nil t) => NIL (and t nil) => NIL (and) => T
Some more practical examples:
> (and T "boo" "hiss" T "rah") "rah" ; return value of AND > (and (princ "hi") NIL (princ "ho")) hi ; prints "hi" NIL ; return value of ANDSee princ.
> (setq a 5 b 6)                 ; set up A and B
6                                ; return value of SETQ
 
> (if (and (numberp a)           ; if A is a number
           (numberp b)           ; and B is a number
           (< a b))              ; and A < B
    (print "A is less than B")   ; then do this
    (print "error"))             ; else do this
"A is less than B"               ; screen output of PRINT
"A is less than B"               ; return value of IF
See also: