CentOS
CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork.) CentOS is free.
Fedora
Fedora is a Linux-based operating system that showcases the latest in free and open source software. Fedora is always free for anyone to use, modify, and distribute. It is built by people across the globe who work together as a community: the Fedora Project. The Fedora Project is open and anyone is welcome to join.
Debian
Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run. Debian uses the Linux kernel (the core of an operating system), but most of the basic OS tools come from the GNU project; hence the name GNU/Linux. Debian GNU/Linux provides more than a pure OS: it comes with over 25000 packages, precompiled software bundled up in a nice format for easy installation on your machine.
Ubuntu
Ubuntu is composed of many software packages, of which the vast majority are distributed under a free software license (also known as open source). The main license used is the GNU General Public License (GNU GPL) which, along with the GNU Lesser General Public License (GNU LGPL), explicitly declares that users are free to run, copy, distribute, study, change, develop and improve the software. Ubuntu is sponsored by the UK-based company Canonical Ltd., owned by South African entrepreneur Mark Shuttleworth. By keeping Ubuntu free and open source, Canonical is able to utilize the talents of community developers in Ubuntu’s constituent components. Instead of selling Ubuntu for profit, Canonical creates revenue by selling technical support and from creating several services tied to Ubuntu.