Since a hard drive typically has two or more platters, each storing data on both sides, the tracks line up on each platter. The identically positioned tracks on each platter are called cylinders. Since typical hard drives are too large to deal with by the track, each track is divided into sectors. Since a track can hold as much as 50 K of data, this would not be practical for storing large files. Sectors are basically slices of the track.
Different drives have different numbers of sectors per track.
Each sector is given an identity during formatting to aid the controller in finding what it needs in the appropriate sector. These sector numbers are written to the beginning and the end of each sector, called the prefix portion and the suffix portion, respectively. These identities take actual space on the hard drive. This explains why there is a difference between the capacity of an unformatted disk and a formatted one. On a floppy, the disk itself can hold 2 MB of data.