A Neutral Look at Operating Systems/Linux
Linux,® sometimes called GNU/Linux, is a free and open source operating system developed by a loosely knit collection of developers around the world. The Linux kernel was first created in 1991 by Linus Torvalds, a Finnish college student. The Linux kernel was built on top of the GNU tools which gave the GNU/Linux OS commonly called Linux. It was designed to imitate the behavior and inner workings of UNIX® while remaining free to all to use.
Linux appears in many commercial operating systems like Red Hat and Novell's SUSE, and also in many non-commercial varieties, like Debian, Gentoo, Knoppix and others. In all, there are hundreds of "distributions", or Linux based operating systems, available for download on the internet, free of charge. Some are discussed in more detail below. If you find a distribution that's not listed below, check if it's based on one that is. These derivatives generally have one or more areas they are intended to be different, but otherwise they will be similar. An extensive timeline graphic for currently functioning distributions and also for those which are no longer active can be found here: futurist.se/gldt.
Linux consists of Open Source software. This means users have the right to receive the source code of the operating system. They are also free to study, modify and redistribute the source code. The GNU Public License is used for about 2/3 of a typical Linux system.
Linux is a result of sustained effort of a huge community of programmers all over the world. Many of them are volunteers working on various projects in their free time. A number of companies also pay developers to work on the software. Linux is available in a variety of different distributions (flavors) including Ubuntu, Redhat, Slackware, Gentoo, SUSE, Mandriva and Debian.
One of the keys to the success of Linux is its open development model. This allows the users to contribute to improving the Operating System. The code submitted by individual developers undergoes a rigorous peer review process. This allows only high quality code into the OS. This review process weeds out bugs and security holes. The huge community of programmers (about a million) are able to fix bugs very quickly. Because of this, Linux is considered a very secure and high quality OS.
Linux is easily the most versatile operating system in the world. In addition to desktop computers, Linux is used in tiny embedded devices, all sorts of network equipment, large servers and even in super computers. In all these environments Linux provides robust performance, powerful networking, and a high degree of reliability. Linux is in general considered more secure than the Windows alternatives. Viruses, worms, trojans etc. are not as prevalent in Linux environments. This is because of a secure design which does not allow any single user/virus to disrupt the whole system. All normal Linux distributions come with solid firewall abilities. Users generally don't have to install anti-virus software. 
Traditionally, Linux desktops have not been considered very user friendly. In recent time Linux desktops have significantly improved. GNOME and KDE are two very popular Linux desktops that feature a very user-friendly environment and plenty of useful software. Some distributions come with all the software that a typical user will ever use packaged with the operating system itself, eliminating the need to purchase extra software as one must do for typical proprietary operating systems. Maintaining and managing Linux desktops can also be very simple.
Linux is free to download in most cases. Product support is available from the commercial Linux vendors and others. Linux often has low TCO (Total Cost of Ownership) because of its increased reliability, low software costs, and low administrative costs. This is particularly true for small businesses that are unable to negotiate low Windows costs, and for the very large businesses that can devote people to taking advantage of Linux. However, Linux is also the subject of a very complex copyright dispute. Service disruptions from viruses and annoyances from spyware are almost unheard of for Linux users.
In recent times Linux has proliferated to cell phones, telecom equipment, data centers and biometrics computation. Linux is expected to become the operating system of choice for consumer electronics and entertainment industries in future.
Linux is a registered trademark of Linus Torvalds.
Live CDs 
People who are entirely new to Linux should try a distribution on a LiveCD which is a full Linux system with video, networking, and sound that will run entirely from the CD and will not modify their current operating system. These live CDs often run slowly but they will give the user an indication of whether that distribution will run on his/her machine, and the user is likely to see that Linux is a user friendly environment with high quality applications, multimedia functions, and network access.
This helps new users to test out how Linux looks before installing it into their system. These live CD also make excellent emergency boot disks, if the system using some other operating systems does not boot up correctly. Many can even install a complete Linux system while running the live CD, making the installation very simple.
Knoppix was one of the first live CDs. It has better hardware detection than most distributions, and many Linux users keep a Knoppix disk since it makes a nice rescue and demo system. Knoppix also allows you to test-drive several popular desktop environments, including KDE, IceWM, Window Maker, Fluxbox, Lars, TWM and Xfce. The book Knowing Knoppix/Knoppix for the first time has more details.
Fedora distributes a popular live CD that uses GNOME and another one that uses KDE.
Distributions for New Users 
People who have chosen to use Linux and need a desktop distribution should try an easy-to-use distro. Each of the following distros help new users by providing a lot of hand-holding. In fact, the default installation will install and configure software such that any new user can immediately become productive.
Fedora is a fast paced development that is updated every six months with new version, although the previous version is kept updated for the following 18 months. The Fedora system is tightly connected with RPM packages, and many open-source programs come ready made for Fedora (such as Mplayer, Gnutella etc...) Fedora is a completely free Linux distribution, meant to be developed with the most cutting edge versions of programs, that are updated with either Yum or Red Hat Networks. Users can choose between the GNOME desktop and the KDE desktop at login time.
Mandriva Linux 
Mandriva Mandriva Linux was the first distro to focus on ease of use.
Puppy Linux 
Puppy Linux is for beginners and runs fast on old hardware. It also takes up little space on the CD, DVD, or USB flash drive. It makes it easy to modify and add to the basic distribution. When one is combined into a disk image it is called a puplet. Users can upload their puplets to the project's website.
Puppy is a single-user system. That is, the user always has root privileges. This is the same poor security model that gets Windows 9x users in so much trouble.
A slightly more difficult distribution that also has low system requirements is Damn Small Linux. DSL has a better security model than Puppy.
Ubuntu Linux 
Ubuntu is based on Debian and produces a new release every 6 months rain or shine. Ubuntu provides free Ubuntu Linux CDs to anyone who asks for them, which makes Ubuntu free-as-in-pizza even if you don't have a high-speed Internet connection. In the default configuration, you cannot log in as root, or use the "su" command to become the root user. Instead of the traditional root superuser, Ubuntu uses sudo to give certain users administrative privileges. Although one may enable the root account, it is not recommended   .
Ubuntu only officially supports a subset of the packages in Debian, but you can access the rest of the packages with a little work. The default setup includes GNOME, but there's also a KDE version of the distribution called Kubuntu.
There are also other versions of Ubuntu available such as Edubuntu (suited for educational use) and the lightweight versions (in early development) Xubuntu and UbuntuLite. Even though the different Ubuntu versions are available as different installations, the only difference between them is what software they have included. So basically they are all the same distribution, and they all have the same software packages available on-line.
Moblin ("mobile Linux internet") is a project developing a version of Linux and applications for netbooks and mobile internet devices.
Distributions for Power Users 
Regular users of Linux, generally called power users, may prefer distributions like Debian or Slackware to the others due to the easy customization and power these distributions give. Due to this, they also make excellent distributions for servers. New users are encouraged to read the extensive documentation available before trying to use these two distributions unless they have some one who is nearby to help them in case of trouble, or if they have some experience in Linux and want to learn more. Both Debian and Slackware have many derivatives, some of which are also for advanced users. Users considering either one should look into the derivatives as well.
Debian GNU/Linux 
Debian is a non-commercial distribution known for its package management system and strict requirements of its packages and releases.
Debian comes in three branches: stable, testing and unstable. New packages to be included in the distribution are initially pushed to the unstable branch. Once the package has been shown not to cause too many crashes it is pushed to testing where it will stay for several months while thousands of users test it and work out bugs. Once most release critical bugs are solved, the package will be a candidate for the stable branch.
An advantage is that the Debian stable branch is very stable indeed and almost bug free but at the price of being a bit outdated. Debian stable often does not include support for the latest hardware and software features desktop users like to see. Therefore, most desktop users of Debian prefer some kind of mix of Stable, Testing and sometimes even Unstable for their final desktop configuration.
Debian also has a very good package management system, quite possibly THE best package management system of any OS in the world.
Debian is also known for being a completely free (as in freedom) operating system, and unless you include the 'non-free' branch in your sources file, everything will be Free Software. The word Free does not pertain to cost (you can sell these things) but to freedom: every package in Debian gives you the right to use, distribute, and modify the software (even for money).
See Using Debian in a Graphic environment.
Slackware is one of the oldest distributions. Its emphasis is on simplicity of design and following Unix conventions. The result of this focus on design simplicity is that configuration is often left to the user. (development source: http://en.wikipedia.org/wiki/Slackware)
Arch tries to provide a stable and functional minimal core of software and then provide a lot of flexibility for users to customise on top of this. Arch operates a "rolling release" policy, so you can install once and then keep up to date, rather than having to wait for periodical releases and then doing a major upgrade.
Source-based Distributions 
Some distributions allow you to build a Linux system starting with just source code. These distributions are not recommended for new computer users, since it is more complicated and everything has to be compiled. Installation is slow, but some people find it worth the trouble, as when installation is completed you are left with a powerful, high-performance system. Additionally it is a great learning experience.
The aim of Gentoo is to provide an up-to-date, customizable, and high performance GNU/Linux distribution.
Gentoo is a non-commercial source-based distribution, which means that when installing programs for Gentoo, scripts will download the source code for the program and then compile it to fit your computer configuration (CPU optimizations). Gentoo uses Portage, a very sophisticated and advanced package manager, to achieve all of this. The inspiration for Portage came from the BSD Ports packaging system.
Installing a complete system can take quite some time, so it is also possible to install Gentoo with pre-compiled packages, at the cost of some speed optimizations. Gentoo users claim that the optimizations and configuration options (USE flags) make their computers noticeably faster, but critics of Gentoo argue that the optimizations are overrated and that the increased speed from optimizations are lost when you consider the amount of time the computer has to spend compiling programs. Also, excessive optimization can lead to instability.
Compared to other distros, Gentoo software tends to be more bleeding edge, which means it is newer and has more features, but is less stable.
Linux From Scratch 
Linux From Scratch is not a distribution at all. Instead, it is an online book that guides a user through the process of downloading, compiling, installing, and configuring each component of a typical GNU/Linux system. Although it tends to be impractical for most desktop users, setting up a Linux from Scratch system can be a great learning experience.
SourceMage is derived from Sorcerer, probably the first source-based distribution with a good package manager. It uses Sorcery, a powerful package manager with full dependency resolution and featuring a lot of innovative features, completely written in bash.
Perhaps the main difference between Gentoo and SourceMage is the ease of use and installation, which is faster because of the project's emphasis on speed.
It's mainly targeted at Linux From Scratch users bored from the process of maintaining their systems manually, and will look somewhat familiar for Debian users, as its package manager shares some concepts with Apt, but is far more flexible. Also, SourceMage has a Social Contract similar to the Debian Social Contract.
See Also 
- "Do I Need an AntiVirus Program on Linux?"; Wikipedia: Linux malware
- "FrequentlyAskedQuestions: DiffGentoo" See Speed under "Other Distinctions"