A Linux distribution is a complete Linux operating system: a collection of free and sometimes non-free software created by individuals, groups and organizations from around the world and having the Linux kernel at its core. Companies such as Red Hat, SuSE, MandrakeSoft, as well as the community Debian project, compile the software and provide it as a complete system ready to install and use.
Linux distributions started to enjoy limited popularity in the mid to late 1990s as a free alternative to the Microsoft Windows operating systems and MacOS on the desktop, mostly among people used to Unix from work or school. It has proven more popular in the server market, primarily for Web and database servers.
The Linux kernel and much of the additional software making up a typical Linux-based system is Free Software; even more of it falls under the somewhat broader definition of Open source software. Like all Free and Open Source software, it is distributed by its maintainers in source form. This form has to be compiled into binary or executable form first before it can be run directly.
A Linux distribution offers compiled versions of the Linux kernel, standard system libraries, and assorted programs that make up an operating system. This makes for an install process similar to other operating systems which are distributed in binary form only (Solaris, Microsoft Windows, etc.)
Distributions are normally segmented into packages, each one holding a specific application or service: one package may hold a library for handling PNG images, another may contain a number of fonts, while a third one supplies a webbrowser.
In addition to just providing packaged compiled code, most distributions offer tools for installation/removal of that are more powerful than simple archiving software: Packages may contain meta-information like description, version, other packages needed for this one, etc. The package management software can evaluate this information, to allow package searches, automatic upgrade to newer versions, checks whether all dependencies of a package are fulfilled up to fulfilling them automatically, and much more.
Typical distributions also incorporate some configuration management, as many programs need to be configured correctly to be useful. A default configuration tuned to the distribution may be provided, or the administrator may be queried for configuration information by means easier than the traditional editing of configuration files.
Although Linux distributions often contain much more software than the typical commercial operating system, it is normal for administrators to install software that is not available through the distribution (or only in an older version). If this software is distributed in source form, this involves compilation as described above.
By replacing everything provided by a distribution, an administrator may reach a distribution-less state: everything was retrieved, compiled, configured, and installed by herself. It is possible to build such a system from the start, but one needs a way to generate the first binaries until the system is self-hosting (has a bootable kernel, and compilation tools to generate more binaries). This can be reached via compilation on another system that is able to build binaries for the intended target (possibly by cross-compilation). See Linux From Scratch Guide for instructions.
General Purpose Distributions
These are the most popular and therefore the most common distributions of Linux, listed approximately in order of their share of the installed base worldwide:
Debian: put together by a horde of volunteers Debian has the biggest selection of packages and is well known for its painless automatic upgrades.
Mandrake: based on Red Hat, Mandrake tries to be the easiest distribution for beginners.
Slackware: one of the oldest distributions still alive, Slackware is quite minimalistic.
Special Purpose Distributions
Some groups compile special purpose Linux distributions as turnkey firewalls, for embedded systems, and for other special purposes.
Special Purpose Linux Distributions:
See the distribution list on The Linux Weekly News.