Software that permanently erases data on a hard disk by overwriting all the sectors with random characters. The normal deletion of a file only frees up the disk sectors involved, but the original data are still there. In fact, there may be magnetic residue from bits that were written several cycles in the past and that have been written over numerous times with new data.
In order to ensure that data are completely unrecoverable on a disk, data shredding software may be able to write over the sectors many times. See hard drive shredder