What is the difference between primary and secondary headaches?

So headaches have multiple causes. As you're pointing out, that there are what we call primary headaches and secondary headaches. Secondary headaches, as the name suggests, is due to something else, having a brain tumour, having abnormal blood vessels in the brain, and a multitude of other reasons. There are at least 300 causes of what we call secondary headaches. Fortunately, secondary headaches are in fact quite rare. The majority of the headaches that occur are what we call primary headaches, headaches such as tension-type headache, migraine. In primary headaches, the problem lies within the brain itself. The problem is best thought of as a software problem of the brain. The pain-sensing system that we have in the brain itself is where the dysfunction is. So there's no structural lesion to be found. You do scans and you do investigations. Those are all normal. But in those people what you find is that despite normal investigations, there is a dysfunction in the pain processing system. So some of these headaches such as migraine, what in essence is going on is that when someone has a migraine attack the pain system is dysfunctional even though there's nothing wrong on the outside. And they end up having a headache because of a problem within the brain itself.