A headache or cephalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the pain-sensitive structures around the brain. Several areas of the head and neck have these pain-sensitive structures,
which are divided in two categories: within the cranium (blood vessels, meninges, and the cranial nerves) and outside the cranium (the periosteum of the skull, muscles, nerves, arteries and veins, subcutaneous tissues, eyes, ears, sinuses and mucous membranes).
There are a number of different classification systems for headaches. The most well-recognized is that of the International Headache Society. Treatment of a headache depends on the underlying etiology or cause, but commonly involves analgesics.
(contd. http://en.wikipedia.org/wiki/Headache )