A floppy disk consists of a flexible membrane covered on one or both sides with similar magnetic substance as a hard disk. The floppy disk itself doesn't have a read-write head, that is included in the drive. A floppy corresponds to one platter in a hard disk, but is removable and one drive can be used to access different floppies, whereas the hard disk is one indivisible unit.
Like a hard disk, a floppy is divided into tracks and sectors (and the two corresponding tracks on either side of a floppy form a cylinder), but there are many fewer of them than on a hard disk.
A floppy drive can usually use several different types of disks;
for example, a 
inch drive can use both 720 kB and
1.44 MB disks. Since the drive has to operate a bit differently
and the operating system must know how big the disk is, there
are many device files for floppy drives, one per combination of
drive and disk type.
Therefore, /dev/fd0H1440 is the first floppy drive (fd0),
which must be a inch drive,
using a inch, high density disk (H) of
size 1440 kB (1440), i.e., a normal inch HD floppy.
For more information on the naming conventions for the floppy
devices.
The names for floppy drives are complex, however, and Linux therefore has a special floppy device type that automatically detects the type of the disk in the drive. It works by trying to read the first sector of a newly inserted floppy using different floppy types until it finds the correct one. This naturally requires that the floppy is formatted first. The automatic devices are called /dev/fd0, /dev/fd1, and so on.
The parameters the automatic device uses to access a disk can also be set using the program setfdprm(8). This can be useful if you need to use disks that do not follow any usual floppy sizes, e.g., if they have an unusual number of sectors, or if the autodetecting for some reason fails and the proper device file is missing.
Linux can handle many nonstandard floppy disk formats in addition to all the standard ones. Some of these require using special formatting programs. We'll skip these disk types for now.