What Causes You To Get Chills Down Your Spine?

I'm sure most, if not all of us have gotten that feeling, the back of your body just feels numb, etc.

I usually get them listening to certain songs that hold meaning to me, and also for some reason I got them seeing the Halo 3 Ad in a magazine, particularly reading the motto thing (if you've seen it, you know what I'm talking about)

this song

http://youtube.com/watch?v=GXUA-Ki6Y84