When most people are referring to headaches, it is a tension headache - a headache caused by muscle tightness across the neck and head.
It is usually an ache which is consistent, and may go across the forehead. Over the counter pain relievers are generally enough to relieve this pain.
Migraine headaches are a neurological problem. During a migraine, there is a spreading cortical depression. This means that there is a slowing down of electrical activity in a part of the brain, and then symptoms are set off, including inflammation of the blood vessels in the brain which cause pain, and also things like temporary visual difficulties, nausea and vomiting, sensitivity to light and sound, sensitivity of the skin, numbness.
Migraine pain is characterized as throbbing or stabbing and it is usually only on one side of the head, although it can be on both. Over the counter pain relievers typically do not help this pain. The OTC pain relievers can in fact make migraine pain worse.