Recovery of supposedly erased data from magnetic media is easier than what many people would like to believe. A technique called Magnetic Force Microscopy (MFM) allows any moderately funded opponent to recover the last two or three layers of data written to disk. Wipe repeatedly writes special patterns to the files to be destroyed, using the fsync() call and/or the O_SYNC bit to force disk access.
Wipe can permanently delete data in hard disks and flash drives (caution! several writes can damage solid medias).
This program is useful in anti-forensics and security activities.