There are really two types of head injuries, one that is an open injury where something actually penetrates the skull and fortunately these are rare other than some industrial or work place accidents or even gunshot wounds. The most common thing we run into is what we call a closed head injury. This simply means the skull didn't fracture, nothing penetrated the skull environment.
Don't be mistaken, a closed head injury can cause serious and permanent long-lasting brain damage. Closed head injuries are actually 75% of all brain injuries. The most common mechanism for a closed head injury is the head shifting and stopping like when you fall and strike the ground or just even in a whiplash motion of the head, moving forward and back again. The brain moves as well, striking the front of the skull and then the back of the skull. This violent crashing around of the brain within the skull can cause all types of nerve damage to the content of the brain as well.
Closed head injuries must be taken seriously. If you have symptoms, you need to understand that you should get them diagnosed and treat them at the earliest opportunity.