Is there a thread that explains how autographs are authenticated?

[pbspelly]

I saw that SGC offers signature verification on-site at some card shows. Forgive my ignorance, but I've never understood how this works. Do they need a previously authenticated item to compare it to? Do they consider the backstory behind the item ("my great-granduncle lived next door to Fielder Jones" sort of thing?) If all the authenticator has is the item to be authenticated, can they do it with just that alone, based on looking at the ink or the item signed?

I'm kind of hesitant to waste your time educating me, so I was wondering if there is already a thread (or website) that does a good job explaining how autograph authentication works.