Jim Freeze wrote:

>   Sorry to be stupid, but how do acceptance tests differ from unit
>   tests?

The short answer is that:

  * acceptance tests are generally specified or written by
    the customer to ensure that the program does what it's
    supposed to do, i.e. it meets it acceptance criteria.
  * unit tests are generally written by the code developer(s)
    to ensure that individual classes or components of the
    software are doing the right thing.

For some more ruminations, google for either "acceptance tests" or "unit 
tests". As usual, Ward Cunningham's WikiWiki 
(http://c2.com/cgi/wiki?WelcomeVisitors) has a lot of interesting pages 
discussing these ideas...