PC-DOS 720K format
The PC-DOS 720K format (3 1/2", double sided, double density) was a very common floppy disk format in the late 1980s and into the 1990s, used on IBM PCs and compatibles. It was the main 3 1/2" disk format for the PC platform until the high-density PC-DOS 1.44M format was introduced. It had 80 tracks per side, with 9 sectors per track, and 512 bytes per sector. Data was stored with MFM encoding. The disk turned at 300 RPM.
These disks were generally used with FAT12 file systems under the MS-DOS or PC-DOS operating system. The disks held exactly twice as much data as the earlier 5 1/4" PC-DOS 360K format, even though the disks were smaller.
The PC-DOS 1.44M format later took over much floppy-disk usage, though the lower-density 720K disks remained in use as well, with the high-density drives supporting both formats (though there could be compatibility issues in reading 720K disks on low-density drives after they were written to with a high-density drive, even though the writing is done in an emulation of the old format, due to the different drive head on the newer drives; this sort of incompatibility might, however, not have been as common as that between the high and low density 5 1/4" disks).
In the late '80s and early '90s, it was common for desktop PCs to have both 5 1/4" and 3 1/2" disk drives in order to be compatible with all software and data, which might be distributed on either format. Often the 5 1/4" drive was drive A, and the 3 1/2" one was drive B. Later PCs, however, were more likely to have only a 3 1/2" drive, set up to respond to both drive letters. Eventually, PCs stopped having floppy disk drives altogether as other data storage and transfer media took over.
3 1/2" disks are actually 90 mm wide, but are almost universally referred to as "3 1/2 inch" disks even in countries that use the metric system.
The Commodore 1581 disk had a sufficiently similar low-level format to allow for software-based transfer and emulation between the two sorts of disks and drives.