There were times when it was not customary to access hard disk sectors by their sequence number. Hard disks (and floppy disks as well) are actually organized in three dimensions. There are several rotating disks, each with two sides (or surfaces). There is one head to read and write data for each surface. That head, or actually the whole stack of heads, can be moved between the center and the rim of the disk. Each surface consists of concentric tracks, which contain the actual sectors. A cylinder is all tracks of all surfaces at a certain radius taken together. Put another way, all sectors in one cylinder can be accessed without moving the head stack. When it comes to coordinates, track and cylinder are interchangeable terms.

The reason you'll find drive geometry coordinates used in partitioning schemes and file systems is twofold: First, older disk interfaces required cylinder-head-sector coordinates because the (simple) drive electronics couldn't handle logical sector numbers. Second, putting data together in one cylinder speeds up access. As hard drives grew more complex and more intelligent, both reasons vanished.