The hard drive is used to save everything, this is the computers memory storage, without it you wouldn't be able to save anything on the computer, NOTHING, not even the desktop background would be saved and not even if you created different users, basically you would have to create a new user every-time you would log on
More information about the Hard Drive
Basically what the Hard Drive also does is it stores your system, so if you didn't have the Hard Drive you wouldn't be able to even load your computer, it would show a black screen with text rapidly scrolling down checking everything works but it would stop when it would get to loading the Operating System and close.
Hard Drive pictures
The basic Hard Drive
The simplest form of Hard Drive, found above aswell
Hard drive colour tinting
Hard drives have disks, these can be bronze or silver coloured.
Hard drive disks
Hard drives have lots of disks inside them