GNU/Linux is an open source operating system that can run on anything from PCs to Macs to Playstations and almost everything in between. Linux itself makes up the nuts and bolts of any Linux Distribution (often called distros). It is the software that runs your hardware. Everything else from the Window Manager to the web browser is typically an open source application that was written by a member or members of the Linux community.
Distros
There are hundreds of Linux distributions around. Following are some of the more common ones, along with descriptions of what they are like:
Ubuntu
The Ubuntu Linux project aims to make Linux accessible to the average user. It is based off of Debian, but many changes have been made, creating a more user-friendly system. Ubuntu is a good distribution for beginners to learn on, as it helps you through the installation process and to learn how to navigate around the system.
Debian
Debian is a very configurable Linux distribution. It has three main branches -- stable, testing, and unstable -- which are all well-maintained. The stable branch does not generally have updates, except for security updates, and a new version comes out every 18 months. To make up for this long stretch of time, the testing distribution is used on most desktops, as it receives many updates, after they have passed through the unstable branch. Debian is a fairly complex distribution, and is most liked by advanced users.
Gentoo
Gentoo Linux is a versatile and fast, completely free Linux distribution geared towards developers and network professionals. Unlike other distros, gentoo linux has an advanced package management system called Portage. Portage is a true ports system in the tradition of BSD ports, but is Python-based and sports a number of advanced features including dependencies, fine-grained package management, "fake" (OpenBSD-style) installs, safe unmerging, system profiles, virtual packages, config file management, and more.
Red Hat/Fedora
The Fedora Project is an openly-developed project designed by Red Hat, open for general participation, led by a meritocracy, following a set of project objectives. The goal of The Fedora Project is to work with the Linux community to build a complete, general purpose operating system exclusively from open source software. Development will be done in a public forum. The project will produce time-based releases of Fedora Core about 2-3 times a year, with a public release schedule. The Red Hat engineering team will continue to participate in building Fedora Core and will invite and encourage more outside participation than in past releases. By using this more open process, we hope to provide an operating system more in line with the ideals of free software and more appealing to the open source community.
Suse
SUSE LINUX is the international technology leader and solutions provider in Open Source operating system software. SUSE's unique expertise in Linux and its largest development team worldwide dedicated to Open Source software has contributed to the recognition of SUSE as the most complete Linux solution available today. With a workforce of more than 500 people worldwide, SUSE has offices throughout Europe, Latin America and in the United States, entirely focused on supporting the Linux community and Open Source development. SUSE LINUX was acquired by Novell, Inc in November 2003.
Mandriva
Mandriva Linux (formerly Mandrakelinux) was created in 1998 with the goal of making Linux easier to use for everyone. At that time, Linux was already well-known as a powerful and stable operating system that demanded strong technical knowledge and extensive use of the "command line"; Mandrakesoft saw this as an opportunity to integrate the best graphical desktop environments and contribute its own graphical configuration utilities and quickly became famous for setting the standard in ease-of-use and functionality. With this innovative approach, Mandriva Linux offers all the power and stability of Linux to both individuals and professional users in an easy-to-use and pleasant environment. Thousands of new users are discovering Linux each and every day and finding it a complete replacement for their previous operating system. Linux as a server or workstation has no reason to be jealous of any other more established operating systems.
Knoppix
Knoppix Linux is the main LiveCD-based distribution. This means that you can burn a CD with Knoppix on it, put that CD into any machine, and reboot the machine into Linux. This is useful when you want to see if a certain hardware configuration will work under Linux without installing Linux on a hard drive, or to fix something that went wrong with an installation, and potentially fix the problem. Knoppix on a CD tends to be too slow for everyday use, as everything must be read off a CD.