It's a lot tougher to say than you want, and there's not a hard and fast medical explanation that's conclusive for migraines. Headache is a catch all term for head pain. Sources are anything from impact, to blood pressure, to muscle fatigue in the neck and dehydration.

Migraines appear to be a problem with blood vessels. It's considered a "neurovascular" problem and affects more than just the head. They can be caused by pretty much the same thing as headaches. They are treated by treating one of the suspected causes. Some people respond well to antidepressants, some people respond well to medicines that affect blood vessels.

Thunderclap headaches are really uncommon, really severe headaches that typically warn about severe problems in the brain, like bleeding or an aneurysm.