Headaches can be from any of a number of sources.  The most common types are:  tension headaches and migraine headaches.  A tension headache is due to the muscles in the scalp and neck tightening up.  This pain is circumferential and band-like.  In contrast, a migraine is due to the blood vessels dilating.  Hallmark signs of migraines are nausea, vomiting, inability to tolerate light, and visual disturbances.

Both types of headaches can be easily treated with a combination of pain medications and anti-nausea agents.  This is often accomplished via intravenous line.

Additionally, severe headaches can be caused by a sub-arachnoid hemorrhage (SAH) which is usually due to a ruptured aneurysm in the brain.  SAH is sudden in onset, often referred to as "the worst headache of my life" and can be associated with loss of consciousness, confusion, or other neurological complaints.  This is a life-threatening condition and needs immediate emergency care.