Disk drives are special devices that allow us to make copies of parts of RAM and store them magnetically. If RAM memory is electronic, think of disk drives as a special kind of magnetic memory. When you save something to a disk, the electronic impulses in RAM are copied and stored to the disk as a series of magnetic impulses. All a disk drive does is translate electronic impulses and magnetic impulses back and forth.
Disk drives are handy because magnetic impulses are more permanent than electronic ones. The disk drive does not require electricity to keep values in memory, so if you store something to a disk, the information will be there when the tidal wave knocks the power out to your computer. (Assuming, of course, that the disk stayed dry and clean) Disks are sometimes thought of as secondary storage for this reason.
There are a number of different kinds of disks. We will discuss a couple of main types: