Reformatting
Reformatting is a high-level formatting performed on a functioning disk drive to free the contents of its medium. Reformatting is unique to each operating system because what actually is done to existing data varies by OS. The most important aspect of the process is that it frees disk space for use by other data. To actually "erase" everything requires overwriting each block of data on the medium; something that is not done by many PC high-level formatting utilities.
Reformatting often carries the implication that the operating system and all other software will be reinstalled after the format is complete. Rather than fixing an installation suffering from malfunction or security compromise, it is sometimes judged easier to simply reformat everything and start from scratch. Various colloquialism exist for this process, such as "wipe and reload", "nuke and pave", "reimage", etc.
Read more about this topic: Disk Formatting