ScanDisk (also known as Microsoft ScanDisk) is a tool in MS-DOS 6.0 and later (and Windows 9x that scans the system for errors, along with being the predecessor to Chkdisk (prounced as "CheckDisk").
it serves the same purpose as ChkDisk, it scans and fixes any errors on the file system. However, Scandisk had a more user-friendly interface than ChkDisk, and has more configuration options, and unlike ChkDisk, this can also fix cross-linked files.
There was a graphical interface in the Windows 9x version, but the original interface was still there if you ran it in MS-DOS mode. It cannot fix NTFS drives, so it wasn't available in the Windows NT systems.
In Windows ME, when you start Scandisk, rather than scanning the system during the booting time, it rather does it right before the login screen comes up.