Can present as acute (<14 days) and chronic (>14 days)
Both types are caused by sudden acceleration-deceleration of the brain with resultant shearing of the bridging veins.
Blood pools between the dura mater and arachnoid
Patients with extreme atrophy are at increased risk (elderly, alcoholics)
Patients less than 2 years old are also at increased risk
SDH are often associated with other brain injuries
Blood pools between the dura mater and arachnoid
Patients less than 2 years old are also at increased risk
Subdural hemorrhage
Intra-axial
Subdural empyema
Hemorrhagic stroke 
Subarachnoid hemorrhage
Traumatic intracerebral hemorrhage
Epidural hemorrhage
Extra-axial