Scientific American answers the question, “What is a headache?”
The most common type of headache is the tension or muscle contraction type, which is frequently caused by spasms in the neck muscles and the muscles of mastication (chewing). This type of headache is usually treated easily by over-the-counter medications. More intense headaches are caused by unknown mechanisms. Most theories of vascular headache involve the relationship between the nerves and the blood vessels, both of which can be sensitive.
eww.. I get the vomiting kind of headaches.. no fun! I was told, however, it was caused by my bad posture and can get rid of it with neck exercises most of the time before it hits with full force. :o)