When to use defparameter and defvar

Because I always forget:

< gigamonkey> Say you were running a genetic algorithm and you
              said (defparameter *populations* nil)
< gigamonkey> Then you run it for a few days and *populations*
              is full of thousands of populations you've
              generated. Then you change some code in the file
              and reload it. Ooops. You've just wiped out your
              *populations*.
< gigamonkey> (defvar *populations* nil) saves you from that.
< gigamonkey> Conversely if you have
              (defvar *mutation-rate* .01) and then you decide
              to tweak things and change the .01 to .001 and
              reload the file, you'll be confused when nothing
              changes.
< gigamonkey> In that case you should have used defparamater.
< gigamonkey> spelled right, of course.