|Just to expand on what DAVEINCAPS said., formatting is the preparation of a storage medium to receive data. Tracks and sector boundaries are laid out and the file system is established.|
With windows a disk has to be formatted before it can show in Windows Explorer and Windows allocates it a drive letter. At this stage the drive will show as empty.
So when people say they want to format their hard disk when they really mean, install the operating system, it can get a bit confusing for those that know what formatting really means.