Comparison of Linux distributions
Wikipedia: Comparison of Linux distributions.
Each Distro has a package managing system, and uses a package format, i.e:
deb for Debian,
rpm for Fedora(Red Hat),
tgz for Slackware,
Package management tools:
Debian: apt, aptitude
Fedora: rpm, yum
OpenSuse: rpm, zipper
Mandrake: rpm, urpm
Gentoo: emerge
Some distros act as base for other derived distros:
Debian (base) -> Ubuntu
Fedora (base) -> Centos
Some distros are backed by an enterprise:
Ubuntu by Canonical Ltd.
Fedora by Red Hat
Most distros distribute binary packages, but Gentoo distribute source code files which you compile in your system.
Linux from Scratch is a guide to build your own GNU/Linux based system.