An abrasion or ‘graze’ is clinically only a minor injury, but may have forensic significance.
It is a superficial skin injury which doesn’t generally penetrate the full thickness of the epidermis, but may bleed when the vessels of the convoluted papillary dermis are damaged.
Occasionally the full thickness of the skin can be abraded.
Abrasions are caused by a tangential 'dragging' or friction force impacting upon the skin - the combination of pressure and movement causes the damaged epidermis to be heaped at the end distal to the site of impact. It may often therefore be possible to deduce the direction of force.