A common type of head injury is from a simple and single hit to the head. It does not damage the brain with exposure and such. Typically, this type of injury is not particularly severe. However, in some instances it can be very dangerous. This would mean repetitive hits or a single very hard hit to the head. Injuries like this are normally partially due to the fact that the victim was not wearing a helmet and other such instances. These kind of injuries can cause concussions.
What happens after the trauma, however, is the interesting part. As you may have heard, neurons do not easily regenerate if at all. If the body of the brain cell is destroyed, the cell will not make a recovery. If the axon is damaged though, it can regenerate. So, if the cell body is damaged, it will not regenerate. This means decreased brain capabilities. Though a post trauma brain is normally operating at a lesser rate than prior to the injury, it can still regain its abilities. If too many cells are lost, the victim may suffer from a permanent loss of brain activity and ability.
It is a sad thing to see this happen to people. It is such an easy thing to prevent though. Simply wear a helmet and protect your head if you plan on doing something that puts your head at risk of impact.