Most headaches are muscular, caused by stress and the tightening of muscles throughout the neck and head that radiate to the forehead, or related to inflammation of various areas of the head and neck.
Migraines are neurologic disorders and not actually classified as headaches. There are various theories as to the cause of migraines and right now there is no clear cut leader. Neuroimaging has pretty much proven that migraines are wholly neurologic, although some people still hold on to the vascular theory of 20 years ago. Generally, there is a seen a decrease or depolarization of electrical energy in the brain upwards of 24 hours before a migraine attack. This triggers a chain reaction that results in a migraine. What causes that depolarization is up for grabs.
I blame the internet and the return of swing music.