http://en.wikipedia.org/wiki/Cathode_ray_tube

Check the Health Danger section if you're worried.

Ohh and DannyMan, I could try and explain why you get headaches when watching TV, but the web does a much better job for me

"The constant refreshing of a CRT can cause headaches in migraine sufferers and seizures in epileptics, if they are photosensitive. Screen filters are available to reduce these effects. A high refresh rate (above 75 Hz) also helps to negate these effects"

Go get your learn on