RUF is affiliated with Porsche, they do basically start with aPorsche and tune it to there own specs, much like Shelby. However, they have thier cars have VIN number different from Porsches from the German Gov, and they are recognized as a seperate automobile manufacturer.
If you have a Porsche and want more, RUF can modify it to RUF specs a lot of the time, though they sometimes can't modifiy it exactly to RUF specs as if you bout a new one.
Click here for more info on RUF
http://www.ruf-automobile.de/