Because floppy disks were developed in the USA, the sizes are still in imperial measurements. They are not in use any more. These disks are getting out of date and used mainly in older systems. Single or double-sided . Single-sided disks are not in use any more.
Density is the number of bytes that can be stored on each track.
As diskettes were developed there have been changes in the density. Originally on DOS computers the density was 1.25 kilobytes per inch of track. Later again further improvements permitted quad density , now commonly called high density disks and drives. The original low density disks have now disappeared, leaving only double and high density disks. This results in some confusion for new users as the name double density seems to imply it holds more than the other current type - high density.