Nearly every desktop computer and server in use today contains one or more hard-disk drives. Every mainframe and supercomputer is normally connected to hundreds of them. You can even find VCR-type devices and camcorders that use hard disks instead of tape.
These millions of hard disks do one thing well: They store changing digital information in a relatively permanent form. They give computers the ability to remember things when the power goes out.
Hard Disk Basics
Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called "fixed disks" or "Winchesters" (a code name used for a popular IBM product). They later became known as "hard disks" to distinguish them from floppy disks. Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.
At the simplest level, a hard disk is not that different from a cassette tape. Both use the same magnetic recording techniques. Hard disks and cassette tapes also share the major benefits of magnetic storage -- they can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years.