http://www.dvla.gov.uk/vehicles/regdocs_whatis.htm
What is a Registration Document/Certificate?
A Registration Document/Certificate shows the registered keeper of a vehicle. The registered keeper is the person who keeps the vehicle on a public road and is not necessarily the legal owner. It gives the keeper's name and address, the registration mark and other information about the vehicle. A new Registration Certificate is issued each time DVLA updates the record with any change to the existing details. You should make sure that the details on this certificate are accurate and you must tell us immediately if we need to make any changes. You may have problems selling your vehicle if any of the information is inaccurate. A Registration Document/Certificate is not a document of title (ownership).