Head injuries are categorized as either closed or open (penetrating).
A closed head injury means you got a hard blow to the head from striking an object, but the object did not break the skull.
An open, or penetrating, head injury means you were hit with an object that broke the skull and entered the brain. This typically takes place when you move at high speed, such as going through the windshield during a car accident. It can also happen from a gunshot to the head.