Category A: Total burnout. No value left in the vehicle
Category B: Break only. Value lies in salvage
Category C: Damage to car is more than total value of car
Category D: Repair would cost almost as much as the vehicle is worth. Better to write off than repair. No comebacks for the insurers.
So basically, a Cat D car was been written off by an insurer. It has then probably been bought of the salvage company and 'repaired' on the cheap. In order for a repaired car to be put back on the road it has to be inspected by a DVLA engineer, who will write a report stating if the car is safe for use, or not. I the engineers report states its safe, you can then put it back on the road, and you then have the option of paying to have it removed from the Insurers database of write offs.
Personally, I wouldn't touch it with a very long barge pole