Reading Audi VIN

Does anyone know the specifics on how to read an Audi VIN. I know the basics about Country it was manufactured, the manufacturer but, how does Audi make use of the 4th to 8th characters? Can you determine whether the cars is Quattro from the VIN? Aside from idle, geeky curiousity, I'm looking for a 2000-2001 TT. I would like to be able to read the classifieds and know what I'm looking at. Sellers often leave out details and I don't want to run a CARFAX on every car I look at.