The floppy disk is a circle of magnetic media encased in a plastic wallet, used for storing data. They can be used to distribute software, transfer data from one computer to another, and for small backups on personal computers.
For many applications they have been superseded. In 2001 most machines used hard disk drives for primary data storage, software is distributed via the internet or on compact disc, transfer of data is done using a network (for example, the internet), backups are made using high capacity tape drives (such as DAT) or writing to compact disk using CD-Writers.
The case in which the disk is held is square (not a disk!) and this can confuse some novice users.
The original floppy disk was the eight-inch. These large disks predate the desktop machine, and were mainly used for transferring data between mainframe machines. Their capacity was extremely limited, by todays standards, with 160 kilobytes available. The reason for the name 'floppy' was that the exterior casing was only soft flexible plastic, leaving them vunerable to manhandling.
Home computer manufacturers, who had previously been using tape (compact cassette) to save data too, saw the possibilities for this medium. Amstrad incorporated a 3" inch disk drive into their CPC664 and CPC6128 models, amongst other manufacturers. They remained expensive and did not become standardised. However, they had a small form factor and a rigid case with a slideable write-protect catch, features which would be seen later in the 3½" disk.
The first floppy disk that was used in home machines was the 5¼". Small enough to fit in a desktop unit, the usual incarnation offered 360kB of storage space. Since these machines had no hard disk, the operating system would have to be loaded on one disk, then removed and replaced by another disk containing the application. Later machines using two disk drives enabled the user to leave the operating system disk in one drive, and only change the application disk. These disks were also flexible, and were usually contained within individual paper envelopes. The much later Quad Density disk provided 1.2 megabytes of information on one disk.
But by this time, the 3½" disk was arriving. Borrowing from the advances made in the three-inch disks, as well as enhanced methods of manufacture, they were able to contain 720kB of data in their first standard, Double Sided Double Density. However, there was soon a new standard - High Density - represented by a human-readable stylised 'HD' in the top right of the disk and a machine-readable hole in the bottom right corner, with the write-protect catch remaining in the bottom-right corner. These were capable of 1.44MB and remain the standard in floppy disks until this day (2001), despite successful attempts to put 2.88MB on a disk via enhanced formatting techniques.
The 5¼ disk had a large circular hole in the centre for the spindle of the drive, and a small oval aperture in both sides of the plastic to allow the heads of the drive to read and write the data. The magnetic media could be spun by rotating it from the middle hole. A small notch on the right hand side of the disk would identify whether the disk was read-only or writeable, detected by an LED below the disk and an LDR above it. Inside the disk were two layers of fabric designed to reduce friction between the media and the outer casing, with the media sandwiched in the middle. The outer casing was usually a one part sheet, folded double with flaps usually spot-melted together. A catch was lowered into position in front of the drive to prevent the disk from emerging, as well as raising or lowering the spindle.
The 3½ disk was made of two pieces of rigid plastic, with the fabric-media-fabric sandwich in the middle. The front had only a small aperture for reading and writing data, protected by a spring loaded metal cover, which was pushed back on entry into the drive. The reverse had a similar covered aperture, as well as a whole to allow the spindle to connect into a metal plate glued to the media. Two holes, bottom left and right indicated the write-protect status and high-density disk correspondingly, a hole meaning protected or high density, and a covered gap meaning write-enabled or low density. A notch top right ensured that the disk was not inserted incorrectly, and an arrow top left indicated the direction of insertion. The drive usually had a button which, when pressed, would spring the disk out at varying degrees of force. Some would barely make it out of the disk drive, others would shoot out at a fairly high speed.