It's to do with Counter-Strike: Source. A demo is footage recorded in-game. You open the console in the menu and type "record *insertname*" (-quotations) and it'll transfer the data to a .dem file located in your CSS directory. You're able to play it back later and whatnot, although that's irrelevent to this topic so can we please stay on it.
Apparently, some faggots took my video off of Filefront so it's down to the megaupload link for now.