Debian From Scratch (DFS) is really two systems: 1) a bootable CD for repairing Linux systems or installing Debian; and 2) the program that generates the CDs that are used for #1.
Read it here
DFS begain as the first installer for the amd64 port of Debian. I also wanted to have a handy rescue disk for systems that use LVM, Reiser4, etc. At the time, the Debian installers didn't support any of this.