When installing a hard disk drive (commonly called a hard drive) for the first time, attempting to remove errors from your drive, getting rid of a nasty virus, or even cleaning a hard drive because you are selling or donating your computer -- those are just a few of the many reasons why one might consider formatting their hard drive.
Format actually means to prepare a storage medium, usually a disk, for reading and writing. When you format a disk, the operating system erases all bookkeeping information on the disk, tests the disk to make sure all sectors are reliable, marks bad sectors (that is, those that are scratched or otherwise damaged), and creates internal address tables that it later uses to locate information. You must format a disk (floppy or hard disk) before you can use it.
Low-level formatting creates the physical structure on the hard drive, a process usually done at the factory. Partitioning divides the hard drive into logical pieces that become volumes. High-level formatting defines the logical structures on the partition and places at the start of the disk any necessary operating system files.
Here endeth todays lesson ...