Volt Cruelerz
Legions Developer
They are very large files, and in my experience, sometimes, they do seem to take forever, but if you leave them in the background or something, eventually they'll start playing..
They store all data between the client and server for a game and effectively replay said game.Literally impossible. The demos contain no real images, sounds or graphics, opposed to other recording programs like FRAPs. It's just player data on each map. This is why they're so small (compared to several gigs from fraps or dxtory, at least).
At least, that's how I think they're made.