A sector is the smallest unit that can be accessed on a hard disk. Each platter, or circular disk of a hard disk is divided into tracks, which run around the disk. These tracks get longer as they move from the middle towards the outside of the disk, so there are more sectors along the tracks near the outside of the disk than the ones towards the center of disk. This variance in sectors per track is referred to as "zoned-bit recording."
Large files can take up thousands of sectors on a disk. Even if one of these sectors becomes corrupted, the file will most likely be unreadable. While a disk utility program may be able to fix corrupted data, it cannot fix physical damage. Physically damaged sectors are called "bad sectors." While your computer may recognize and bypass bad sectors on your hard disk, certain bad sectors may prevent your disk from operating properly. Yet another good reason to always back up your data!