PCs used two types of floppy disks. The first was the 5.25" floppy (diskette), which became ubiquitous in the 1980s. It was superseded by the 3.5" floppy in the mid-1990s.
5.25" Flexible Jacket
Very bendable in its plastic jacket, the first 5.25" floppy held 360KB. After it was upgraded to 1.2MB in 1984, 360KB diskettes were still used to distribute software, because 1.2MB drives supported both formats.
3.5" Rigid Case
The 720KB 3.5" floppy was introduced on IBM's Convertible laptop. Capacity doubled to 1.44MB with the PS/2 line, and 1.44MB drives supported both media. The visible difference between the two was that the 1.44MB diskette had a hole in the upper left corner. IBM offered an extra-high density 2.88MB drive on selected PCs that was backward compatible, but the format never caught on.
Floppy Disk Formats
720KB 3.5" DS/DD
1.44MB 3.5" DS/HD
2.88MB 3.5" DS/ED (IBM only)
360KB 5.25" DS/DD
1.2MB 5.25" DS/HD
DS = double sided
DD = double density
HD = high density
ED = extra=high density