A MTB helmet generally will have a visor, removable or adjustable. MTb helmets also tend to fit lower down the back of the head giving extra coverage, and are more likely to have an in moulded construction, which is a fancy way of saying that the polystyrene inner is covered all over in outer plastic shell. Google six six one recon or giro xar for examples
That said, some helmets claim to fit lower but don't ....
Road helmets tend not to have visors, fit higher up on the head.