A typical 3.5in Floppy disk internal drive.
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 today’s standards, with 160 kilobytes available. The reason for the name 'floppy' was that the exterior casing was only soft flexible plastic, leaving them vulnerable 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, among other manufacturers. They remained expensive and did not become standardized. However, they had a small form factor and a rigid case with a slidable 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. Amiga floppies carried 880k. However, there was soon a new standard - High Density - represented by a human-readable stylized 'HD' in the top right of the disk and a machine-readable hole in the bottom left 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 (2002), despite successful attempts to put 2.88MB on a disk via enhanced formatting techniques. Microsoft applications were often distributed on 'Microsoft distribution format' disks, a hack that allowed 1.68MB to be stored on a 1.44MB disk by formatting it with 21 sectors instead of 18. The most often used file system on floppy disks is FAT12.