Well, most video cards output DVI-I, which has an analog signal with it. So, the DVI-D to VGA (dsub-15) just takes the analog signal and puts it over the pins necessary for a normal VGA plug.

DVI has a better picture to me, though.
Some video cards (the Intel GMA900/GMA950 via ADD2-N) only ouput DVI-D though, so these adapters won't work with them. But most cards do use DVI-I.