There is no difference — both terms refer to the same device inside your computer’s case.
When it comes to understanding computer disks, the easiest analogy to use is a plain old audio cassette. Computer disks are coated with the same magnetic stuff as the tape in a cassette, except on both sides, and magnetic patterns are recorded in order to be played back. The idea of how they operate is the same, but the terms used are different. When we talk about cassette tapes, we use the terms "record" and "playback," but when we talk about computer disks we use the terms "write" and "read."
A floppy disk is so called because the coated disk inside the plastic case is flexible. With floppy disks, the read/write heads of the disk drive actually touch the spinning disk, just like the heads in an audio cassette recorder. For this reason, floppy drive heads need to be cleaned periodically, just like cassette heads. You can’t just use Q-tips and alcohol on the heads, however, because you can’t reach them. A special cleaning device looks like an ordinary floppy disk but contains a disk-shaped pad inside the plastic case, onto which you put a few drops of cleaning fluid. You should clean the heads on your floppy drive about every six months, or data loss can develop.
But what about a hard disk? Well, remember the old record-stacking turntables? Just imagine a turntable that not only would let you stack a number of records, but also had two tonearms for each record, one for each side. Then you could instantly play any song on any side of any record. Now if you substitute floppy disks for the records and read/write heads for the tonearms in our imaginary device, you have the basic idea behind the hard disk. Yes, that’s right — the hard disk in your computer is like a jukebox for floppy disks, except that the actual platters in a hard disk are rigid, not flexible. The read/write heads actually look like miniature tonearms, but they do not physically touch the disks — they just come very close.
One last comment: the computer industry has decided to spell the magnetic variety "disk," with a K, while optical or CD-ROM media is referred to as a "disc," with a C.
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