When a hard disk undergoes a low-level format, it is divided it into tracks and sectors. This low-level format only happens once in the drive's life before it leaves the manufacturer's factory. All subsequent formatting of hard drives derive from this initial low level format, which contains information on tracks, sector layout, defect management and so on.
The tracks are concentric circles around the central spindle on either side of each platter. Tracks physically above each other on the platters are grouped together into cylinders which are then further subdivided into sectors of 512 bytes apiece.
The concept of cylinders is important, since cross-platter information in the same cylinder can be accessed without having to move the heads. The sector is a disk's smallest accessible unit. Drives use a technique called zoned-bit recording in which tracks on the outside of the disk contain more sectors than those on the inside.
