3½" floppy diskette
When you disassemble a floppy diskette
you will see the plastic filmy surface upon
which your data is electronically stored.

hard disk drive
This same platter (disk) shape upon
which data is electronically stored can
also be seen in a hard disk drive.

When a disk is formatted (or initialized), the Disk Operating System places tracks, sectors and bytes in a specific pattern upon the disk.

Tracks are concentric (having the same center) circles placed upon the disk.  Sectors are subdivided sections of each track (in red).  Bytes are further subdivisions of the sectors (in blue).

The number of tracks, sectors and bytes is unique to the specific operating systems.  For example, the number of tracks placed on a disk from a Macintosh operating system is different than that of a PC operating system.

These tracks, sectors, and bytes establish the addresses by which a computer will store information on a disk.  Think of it like your home address.  Your house is a byte.  The block upon which you live is a sector.  The street upon which you live is the track.  Just as you can find your home knowing this information, the computer can "remember" where it has placed its information.