What is "Flash Linux"
It's a FREE (GPL-2) customised Linux distribution initially designed to be run directly off a 256Mb USB key or other (similar) forms of bootable flash memory. It has subsequently (also) become a Linux distribution that runs directly from a CDROM, typically known as a Live-CD.
Flash Linux is based on and new versions / bugfixes should track the stable Gentoo tree. Whereas Gentoo is a source distribution, this is a binary only distribution. If you need the source code or want to make modifications - please take a look at the Gentoo home page.