To speak colloquially on reformatting it actually erases the entire data from the disks for the new upcoming datas. Since the data varies between different operating systems reformatting also varies accordingly.
Reformatting refers to the high level formatting on a storage space which is to be freed. Before gaining knowledge about reformatting the difference between the high level formatting (reformatting) and low level formatting (formatting) need to be briefed.
The main difference:
The low level formatting normally refers to the process done to initiate a hard drive for storage and file processing purpose. But high level format refers to, formatting a drive containing data. Here the previously available data has to be truncated and also the process needs to check and correct the internal errors. Plus additionally it has give support to file system conversions as well.
It is also widely spoken that reformatting means completely wiping the drive and replacing it with junk values without leaving any track of information. But this is not accepted because data lost after reformatting can be recovered, so fact of data getting replaced with junk data can be said as a myth.
What is Reformatting?
Reformatting is the process of not only formatting the storage drive (i.e. removing the data from the drive), but also changing the present file system to another. For example FAT to NTFS, FAT to ex-FAT etc.