In short
A small forward-facing camera mounted in the car; useful evidence in collisions but not allowed to film during a UK driving test.
A dash cam is a small forward-facing camera mounted to the windscreen or dashboard that records the driver's view of the road. They are widely used for collision evidence.
Dash cams that face the driver (recording inside the car) are not permitted during the UK practical driving test — examiners require them to be covered or removed. Externally-facing dash cams are fine.