Fedora is an RPM-based, general purpose collection of software, including an operating system based on the Linux kernel, developed by the community-supported Fedora Project and sponsored by Red Hat. The Fedora Project's mission is to lead the advancement of free and open source software and content as a collaborative community.
Red Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide.
Debian is a computer operating system composed of software packages released as free and open source software especially under the GNU General Public License and other free software licenses. The primary form, Debian GNU/Linux, which uses the Linux kernel and GNU OS tools, is a popular and influential Linux distribution.
Ubuntu is a computer operating system based on the Debian Linux distribution and distributed as free and open source software. It is named after the Southern African philosophy of Ubuntu ("humanity towards others").
CentOS is a community-supported, mainly free software operating system based on Red Hat Enterprise Linux (RHEL). It exists to provide a free enterprise class computing platform and strives to maintain 100% binary compatibility with its upstream distribution. CentOS stands for Community ENTerprise Operating System.
qmail is a mail transfer agent (MTA) that runs on Unix. It was written, starting December 1995, by Daniel J. Bernstein as a more secure replacement for the popular Sendmail program. qmail's source code is in the public domain, making qmail free software.