[plt-scheme] Is there a general overview of contracts somewhere?

Looking at Chapter 13 in the MzLib manual is enough to make me curious
about contracts ... but that manual is a bit too terse for me: I'm not
sure just what problem they solve, nor how to use them.  Googling for
"mzscheme contracts" didn't bring up much of use (I really don't feel
like reading a doctoral thesis just to learn how to use them).  So: is
there something I could read that would introduce the idea of
contracts, and show some examples of their use with MzScheme?

Thanks
-- 
Always code as if the guy who ends up maintaining your code will
    be a violent psychopath who knows where you live.  John F. Woods