Raoul, I think he's referring to CarFax
Carfax is actually fairly thorough, depending on which state you live in and whether or not emissions testing is manditory.
What a carfax will tell you is the:
-Date of 1st registration.
-Any emissions tests as well as what the mileage was at the time.
-How many times the vehicle has been registered (sold) and what the mileage was.
-Whether or not it has been in a major accident (salvage title) or flood.
CarFax is a good idea whenever you are looking at a vehicle. Not only are you able to compare mileage records, but you also know how many owners it's had, how long they owned it and whether or not the title is branded.