OBD= On Board Diagnostic... II = Version 2?
(there is an OBDI, but I beleive all Porsches, and other brands(?) have used OBDII since 1996)
It is a simple computer plug in that allows one to download all the fault codes and "information" from the cars onboard computer. Tracks all kinds of stuff, like how many times you hit redline, for instance. Useful in diagnosing todays cars.
It is my understanding that a dealer can only reveal previous information if you have the previous owners release it. A bit like Dr/Patient confidentiality. That said, if the car has always been local they might still be able to look at the info, just not tell you what it says. I don't beleive PORSCHE's computer system would allow, say a dealer in Cali look up info on a car in Pa.