[plt-scheme] Another question: eq? and eqv?

I understand that eq? is supposed to compare objects for identity (like
comparing pointers) and equal? compares objects for value. I also
understand that equal? recursively applies eqv? to the components of a
structure. What is less clear to me is how eq? and eqv? differ. When is
(eq? a b) different from (eqv? a b) ?

===
Gregory Woodhouse  <gregory.woodhouse at sbcglobal.net>
"Interaction is the mind-body problem of computing."
--Philip L. Wadler