Brain injury can result from a number of causes. Trauma is a leading cause of brain injury (traumatic brain injury-TBI), whether it occurs from a car collision or fall event. Sometimes brain injury can result from a chemical exposure, or oxygen deprivation through birth or a drowning event. TBI can cause a host of physical, cognitive, social, emotional, and behavioral effects, and outcomes can range from complete recovery to permanent disability or death. Wikipedia contains an informative discussion of brain injury.