Linux - an implementation of the Unix operating system,  has become extremely popular in the recent years, and is being cited as the future de facto standard of the industry . With major vendors like IBM, HP, Cray providing the option of Linux as an operating system on their new products, and with the plethora of commercial as well as free software  available on the platform, it really seems that the future belongs to Linux.  The ever increasing popularity of  Linux can also be gauged by the fact that as per a survey conducted by the Linux counter site , there are between 14 - 29 million  linux users worldwide.  Estimates obtained in February 2005, by counting teoma and google hits show that there were  269 million hits from Linux+linspire, as opposed to 162 million from all versions of Windows put together,  27 million from solaris and 62 million from BSD.

Linux - as the operating system is commonly known, is in fact the kernel written by Linus Torvalds, which was first used in the GNU operating system written by the Free Software Foundation founded by Richard Stallman.  In 1984, the Free Software Foundation started working on a Unix-like operating system that would be composed entirely of free software.  The kernel for this operating system, the GNU-Hurd, was still in it's infancy in 1991, when Linus Torvalds released the first version of the Linux kernel .  Since the GNU operating system was otherwise complete, save for the kernel, GNU decided to use Linux as the kernel for the GNU operating system, and a complete operating system GNU/Linux was launched.

Linux distributions started appearing  soon after the Linux kernel was first used by individuals outside the original Linux developers.  The original  GNU/Linux developers were primarily focuused on developing the operating system. Distributions provided user application programs, graphical user interfaces for configurations and additional softwares,  to make it a complete product from the user's point of view.  A linux distribution, informally known as Linux distro, is a unix-like operating system comprising of the Linux kernel,  the GNU toolchain and assorted free, opensource and occasionally proprietary software.  Unlike most other operating systems, the individual components e,g the kernel, the core libraries, startup scripts, shells, GUI tools etc, are available separately for Linux. Each distribution can therefore take some set of components and package them together along with some custom installation routines, thereby creating different working Linux systems. The end result is that, each distribution has its own personality and some distributions are better suited to some tasks than others.  Distributions are normally segmented into packages, each holding a specific application or service. So they contain packages for viewing and editing images in different formats, packages containing different fonts, web hosting services etc. However, it should be noted that by replacing everything provided in a distribution, an administrator may reach a distribution-less state where the system is built from scratch, bypassing any distribution altogether.

Early distributions include MCC Interim Linux, which was introduced in 1992 and kept in the ftp server of the University of Manchester for free download, TAMU - created by individuals at Texas A&M university and SLS - Softlanding Linux System. None of these distributions were well maintained. So Partick Volkerding released a distribution based on SLS, which he called Slackware - and is the oldest distribution still in active development.  Companies like Red Hat, SUSE and Mandriva (formerly Mandrake) and community projects such as Debian, Ubuntu and Gentoo Linux are some of the more commonly known distributions. There are over 300 Linux distributions in active development.

Distributions vary on several dimensions such as:

• Package management -  In addition to providing packaged compiled code, most distributions offer tools for installation/removal of packages. This software is called the Package Management System(PMS). Each package intended for such a PMS contains meta information like description, version, dependencies on other packages or libraries.  Red Hat uses the Red Hat Package Manager (RPM) which is now used by many other distributions as well. Debian uses APT (Advanced Packaging Tool), . tgz or tar.gz was originally used by Slackware and now used by many. ebuild - a source primary install contains mechanism to retrieve compile and install a package in Gentoo Linux's Portage system, using the command 'emerge'.
   
  
• Commercialization -  Most Linux distributions can be downloaded freely or obtained from cheap  CDROMs for Linux retailers. However some distributions are purely commercial, while some like Red Hat and Suse have both free and commercial versions.
   
  
• Target audience -  Some, like Debian, are aimed at experienced system administrators. They do not contain much in terms of GUI based system administration tools or user friendly installation routines. Others like Lycoris, are aimed at Linux newbies and have easy installation routines and GUIs. Some like  Dynebolic and Knoppix are aimed at Windows users. Engarde secure is specifically for mission critical servers.
   
  
• Platform architecture - Most distributions are ported on x86, while some are additionally ported on Sparc, PowerPC, IBM mainframe, AMD etc.
   
  
• Media - Live CDs, bootable keydrive or hard disk installations
  

Some disk based distributions are :  
 

Arch Linux - a i686 optimized, lightweight and flexible general purpose linux system intended for competent linux users ( no configuration helpers)
 

Asianux - jointly released by Red Flag Software, China, Miracle Linux in Japan and Haansoft in Korea, to promote a single unified Linux standard in Asia.
 

Aurox - a distribution that emphasizes support for hardware, specially laptops and for European localizations
 

CentOS - a enterprise-class linux distribution built from sources freely made available to public, by a North American Enterprise linux vendor
 

Debian - a community based free distribution based on GNU/Linux, which serves as the basis of other distributions like Knoppix and Ubuntu Linux
 

DeMuDi - developed by AGNULA ( a GNU/Linux Audio distribution) as a free distribution based on Debian and devoted to professional and consumer audio applications and multimedia development
 

Fedora - descendant of Red Hat linux, that is an openly developed project designed by Red Hat, with participation from the Linux community
 

Gentoo Linux - famous for its extreme adaptability, embodies a technology called portage, that allows it to become an ideal secure server, embedded solution, development workstation or gaming system
 

Gobolinux - a rootless linux distribution that breaks with the historical unix directory hierarchy and allows one to build a linux distribution within one's home directory
 

ImpiLinux - a free South African Linux distribution which has now been taken over by Ubuntu Linux
 

Mandriva (formerly called Mandrake) - created in 1998 in an effort to make Linux more user friendly by incorporating graphical desktop environments and graphical configurational utilities
 

Mepis - originating in USA, a desktop Linux system that can also function as a dedicated server. It contains tools for NTFS partition resizing and has recovery CD
 

PLD Linux - a RPM based distribution made in Poland by the Poles which incorporates, among other features, preparation for automatic updates including a feature for restarting services when they are updated
 

Scientific Linux- enterprise Linux recompiled from source. Put together by Fermilab, CERN and some other labs around the world, it allows easy customization and has additions like Pine, openAFS
 

Slackware - first released in 1993, the project has aimed at producing the most UNIX-like Linux distribution that aims at simplicity and stability.
 

SourceMage - a recent source based GNU/Linux distribution
 

T2 - a system development environment that allows creation of custom distributions. With T2 one can define targets for various purposes, ranging from embedded linux systems to a full desktop system featuring X.Org foundation, KDE, Gnome, OpenOffice.Org. The targets can be compiled for use on architectures like Alpha, ia64, PowerPC, Sparc, x86/x86-64 etc.
 

Ubuntu - a distribution that derives it's name from an ancient African word meaning humanity to others and is a freely available distro with both community and professional support.
 

Vine - a Japanese distribution with integrated Japanese environment for desktops and notebooks
 

Linspire - previously known as LindowsOS, is a GNU/Linux distribution based on Debian, with a focus to building a easy to use system catering to the average person
 

RedHat - Linux distribution with enterprise, desktop and academic editions and available for different architectures like x86, AMD64, Itanium, PowerPC, IBM mainframe etc.
 

SUSE - a community project that started in Germany and is now sponsored by Novell with a goal to making it the easiest linux for anyone to obtain and the most widely used distribution.
 

Xandros - a linux distribution that is being projected as the most effective replacement for Windows desktop. Among other things it has wizards to setup virtual private network, firewalls and can encrypt home folders.
 

LiveCD distributions, from which the system can be booted to work like a full-fledged Linux installation without having to install the OS in hard disk, are
 
 

Dynebolic - a live CD which allows editing and broadcasting of sound and video (multimedia production), without the need to install anything on the hard disk
 

PCLinuxOS - an English only live CD that runs from a bootable CD. However, one can also install it on the hard disk with a easy-to-use live CD installer.
 

Knoppix - a bootable live CD can be run as a productive desktop, as well as a rescue CD or used as a platform for commercial software product demos
 

Wolvix - a desktop oriented live cd for regular as well as advanced users
 

ATmission - a live CD which allows one to modify any file on the ATmission CD, implying that one can create user accounts, install additional RPMs etc
 

Xenoppix - customized Knoppix that includes a virtual machine monitor Xen.


Some recent releases are Frugalware linux - a general purpose distribution,, Puppy linux that will install to USB, live-cd, Zip or hard drive media, Tilix - a Bulgarian distribution based on Knoppix etc.

Among the hundreds of linux distributions, only a handful get media attention, and fewer still, become household names in the Linux community.  Rankings in Distrowatch.com show a fairly stable listing with the order of rankings hardly changing much over months.  Some of these are special purpose operating systems that have been optimized for certain functionalities.  SME server (introduced as e--smith in 1999)  was designed primarily as a  plug-and-play file server and network gateway.  SmoothWell Express -  an opensource firewall distribution based on GNU/Linux was meant to convert a PC to a dedicated firewall and gateway.  Kanotix - a variant of the more popular live CD Knoppix has been optimized for modern hardware and has additional scripts for hardware detection and configuration along with improved performance speed. But even among the general purpose distros, one can choose between an easy to install distro or sophisticated but terse one, between one with plenty of user applications of a particular kind or a bare-bone one where the user himself installs the packages required by him, one that gives a Windows look & feel versus a traditional desktop system. Whatever be the requirement, there are several distributions both commercial and free, that will cater to it and it is this gamut of options that makes Linux so unique. Linux newsgroups are a great form of support for all distributions of Linux, though, there is better support for the more popular versions rather than the rare ones.


Acknowledgements - Material for this document has been collected from distrowatch.com, en.wikipedia.org and several internet documents related to linux distributions.