What exactly is Stereotactic Radiosurgery?

So, I was just chatting with a buddy of mine and we got to trying to define what was going on.  So, since we were online, we naturally called up youtube.  First, I'll show you the one I'm going through.  Then, there is an amazing video that  shows the next generation, and it's GammaKnife® - yes, with the registered trademark, that shows some more detail in a computer animation about what is going on inside the machine.  (There is a video in the related videos list of a woman's treatment that was very successful, but was an older version of the technique and it looks like it caused her a vast bunch of pain and discomfort.  I'm not posting that here, but I'm not having to endure that, though it looks like it was worth if for her.)

This is video from a different institution, but the machine looks familiar.  You will see a box put over the patient's face.  My head is too big, so they're not using that box, but the are using the mask.  You'll see what I mean.  At the place I'm doing it, they have a Pandora One account, so I shared a station I made and I can listen to it while the treatment is actually going on. :D

Below is the future of the technique, probably the present in some more affluent part of the country.  Pretty awesome.  (Nice music and video, too!)