[Daniel is a little puzzled that that's what she gets confused about, but he tries to explain anyway.] A computer system is a machine primarily used to store information, such as text, schematics or other things. They can also be used to communicate over long distances if you've got a connection between them, kinda like our eyepieces. Though now that I think about it, these probably run off some sort of magic. Computers are generally powered by electricity, though I have seen some powered by other means. The computer I built for that ghost needed an alternate power supply as well, because all the time it spent being incorporeal kind of made connecting it to a powerpoint somewhat inconvenient.