I am a migraine sufferer. I was about 16 when I started to have these crazy visual field disturbances that were followed by a crippling headache, but I didn't know what was going on. I didn't really talk to anyone about it and just sort of hoped they would go away. Depending on the time of year and amount of stress in my life, they only happened every few months, so it didn't seem bad enough to go to the doctor (ah, the foolishness of youth). Then in my 2nd year of university I was reading "Fundamentals of Neuropsychology" (a great text by Kolb and Whishaw) and I saw a picture of the strange pattern I saw right before my headache started (shown below). This visual pattern appears small at first but it grows in size. It is very annoying. Then it disappears and the headache starts. I read on and learned that I have "classic migraines", which are vascular headaches (unlike normal headaches, which tend to be caused by muscle tension in the head and neck areas). Classic migraines start (most often) with an "aura". The aura is the sensory disturbance that comes before the headache. There are different kinds of aura, but mine is a visual aura, called a "scintillating scotoma" (pictured here).
There are a few theories on what causes the scotoma. The first one I read about is the "vascular theory". This theory states that blood flow to the visual cortex of the brain is decreased, which causes the disruption in vision (i.e., the scotoma). After a short while (maybe 10 - 60 minutes) the blood flow is restored and vision normalizes. Not only is blood flow restored, however, but it is greatly increased. This increase in blood flow causes the stretching of blood vessels in the brain and this is what is thought to cause the headache pain. Remember, brain cells don't have pain receptors, but blood vessels do. When they stretch, they cause a lot of pain.
There is another theory on what causes the aura. It is called the "spreading depression" theory, and it seems to be the most well accepted theory now-a-days. Spreading depression is a seizure-like event that slowly spreads over the surface of the brain. It leaves neurons silent ("depressed") in its wake. This lack of activity in the neurons, when spreading depression affects the visual cortex, leads to the visual field disturbances. This spreading depression also causes the release of nitric oxide from neurons. Nitric oxide is one of the body's most potent vasodilators (something that causes blood vessels to dilate). [Aside: this is why patients with heart disease are often given a form of nitric oxide to take during an angina attack -- it opens the blood vessels and stops the angina]. The dilation of blood vessels causes the pounding headache associated with migraines.
Many of the treatments for migraine (e.g., the triptans - like sumatripan, aka Imitrex®) work by binding the serotonin receptors on the blood vessels, causing the blood vessels to constrict, therefore stopping the pain caused by vasodilation.
I have taken triptans a few times. I know that those who have frequent, extremely painful migraines like to have a stash of triptans around. However, I'm fairly lucky insofar as my migraines aren't terribly frequent (1/month, on average) and I've developed a fairly good system to live with these nasty headaches without having to buy the expensive drugs.
First off, I can lessen my chances of getting a migraine by controlling my stress levels, drinking less coffee, avoiding moving from really dark rooms to really brightly lit rooms and avoiding those annoying, flickering fluorescent bulbs. Other triggers are out of my control, however, such as big swings in the weather.
Another trick I've learned is to use the aura to my advantage. The aura, although incredibly annoying, warns me that I'm going to get a really bad headache. The gives me 15-45 minutes (usually the duration of my auras) to get some pain killers in me. I discovered the miracle of liquigel ibuprofen (very fast acting). As soon as I get my aura, I'll take a few of those puppies and I find the pain isn't nearly as bad. In general, the bigger my scotoma (i.e., the more messed up my vision gets) the worse my headache will be. If it's really bad, I'll take my liquigels and I'll go home and try to sleep it off in a dark room. This isn't always possible, but it is a sensible way of dealing with the pain. Generally, after a migraine I'll have a sore head for a day or two. Coughing, sneezing, fast turns, etc. will cause my head to pound, but it's manageable.
I know this post wasn't about epilepsy, but there is increasing evidence suggesting a link between migraine and epilepsy. Is migraine a form of seizure? Certainly, the "spreading depression" theory suggests that it is. Fortunately, however, migraines do not lead to a loss of consciousness and can be managed fairly well with drugs/lifestyle changes. Whether or not you need to take prescription drugs chronically largely depends on the frequency and severity of your migraines.
Also, there are many kinds of migraine. I've only discussed the "classic" form here. Perhaps I'll post about the others later.