(LFS) - Linux From Scratch is a Linux distribution that you build from source (scratch) piece by piece.

(LFS) - Linux From Scratch is a project that provides you with step-by-step instructions for building your own customized Linux system entirely from source.

The process is time consuming and complicated but, it offers you insight into exactly what is required to make a working system, how those components work together, and provides you with an extremely "lean" system devoid of extra software.

This is essentially a learning tool to extend your Linux knowledge and understanding of the Linux internals.

LFS: Home Page
LFS: Wikipedia entry

Once you build your LFS machine it won't do much other than be a bare-bones Linux system. It will not have a GUI, play movies, or download torrents. It will simply provide you with a base. Then should you wish to learn the next stage or progress you will move on to BLFS: Beyond Linux from Scratch, HLFS: Hardened Linux From Scratch, or another source based distro.

Other Source based distributions:
Calculate Linux
Lunar Linux