A sector is an area on a PC disk (hard disk or floppy disk) used to store data. Sectors, which resemble the slices of a cake, are laid down on the disk when it is prepared for use, or formatted. The size of each sector varies depending on the operating system and is defined in the disk’s boot sector.

A disk is also divided into cylinders (or tracks) and heads (or sides). Data on a disk is accessed, at a low-level, according to its cylinder, head and sector number. Of course, the user doesn’t need to worry about this low-level information, since the operating system handles the storage and retrieval of data in a user-friendly way.