Track0 (pronounced 'track zero') is the area located at the start of a computer disk and is predominantly used to store information regarding the layout of the disk (the partition table) and executable code needed to boot an operating system. Track0 typically consists of the first 63 sectors of the disk (since each sector is normally 512 bytes, this corresponds to 32256 bytes = 31.5KB).
The first sector of Track0 is known as the master boot record (MBR) and contains the initial code used to boot the operating system (bootstrap code). Near the end of the MBR is the Partition Table: a predefined structure containing the layout of the disk.
Track0 is also increasingly used to store licensing information for particular pieces of software because it is an area of the disk outside of the OS filesystem and not seen or used by most computer users (and therefore cannot be easily modified or replicated).
The origins of Track0 came about from the original specifications of PCs, where computer hard disks were defined in terms of Cylinders, Heads and Sectors (CHS system). In this context, Track0 was defined to be all the sectors on the first cylinder and first head. As with many (now legacy) original PC specifications, the terminology stuck and the first 63 sectors of modern hard disks are still referred to as Track0.
Track0 is also the name of a terminate-and-stay-resident DOS program that allows users to revive floppy disks with defective first tracks by swapping it with another track under the hood. With DOS, the first track of a floppy disk is used to store system information. If the first track is defective, the whole disk becomes unusable. Track0 tries to work around this limitation.
While it was the first program of its kind, Track0 entered the game when the use of floppy disks had already declined. Even so, a plethora of similar programs appeared shortly after the release of Track0.