The concept of “distros” and the incredible number of different distributions available might be overwhelming to the new user. To help understand, explore and chose a distribution, I will show here an overview and some short explanations of such distributions.
A distribution of GNU/Linux is a set of selected software, organized in a certain way, designed to fulfill a specific need. A distribution is often a complete operating system that is ready to be used. Some of the most popular distributions are: Debian, Ubuntu, Mint, Fedora, Red Hat, SUSE... and so on. There are more than 300 distributions listed in distrowatch.com today. No wonder that the task of choosing a distribution seems quite daunting! Don't panic, it's not as chaotic as it might seem.
There are actually only a few important distributions and a lot of distributions that are based on them. The three most influential distributions are: Debian, Fedora and Slackware. Almost all distributions out there are based on one of these three! There are also three major commercial distributions that benefit from a lot of manpower from dedicated professionals and are deployed in a large number of enterprise-level platforms. It's not a surprise that these distributions are derived from the three distributions cited before: Ubuntu (backed by Canonical Inc. is based on Debian), RHEL (backed by Red Hat Inc. is based on Fedora) and SUSE (backed by Novell Inc. is based on Slackware). But what does it mean to have a distribution “based” on another? Let me explain further.
All Free Software can be used by whoever wants to use it. There is a huge amount of Free Software available and it is constantly evolving. A distribution will pick some of these software and create a specific repository of them (something like a software warehouse). The repository of any distribution is a sub-set of all available Free and Open Source Software. When I say that a distro is based on another it means that it will build itself on top of the choices already made by another distribution.
To make a comparison, imagine that, in a supermarket, you can buy a flavour of ice cream or a box with several flavours. The box with several flavours is a “distribution” of the available flavours. Someone can also make a box containing several boxes of several flavours, based on the available boxes of several flavours. The second box is “based” on the boxes of several flavours. Still with me? So let's get back to GNU/Linux!
To make things clear take a look at the tree below. These are the 50 most popular distributions today and their relationship to one another. All of them use software from GNU and the Linux kernel so they all share these software repositories. At the right are the most influential distributions and their popular derivatives. At the left are independent distributions that are also among the 50 most popular distributions according to distrowatch.com. The top 10 distributions are written in bold letters. From this tree you can easily see that some distributions create a lot more synergy than others.To chose the right distribution for your needs, you have to consider a few things. The most important one is to know what you want from your computer. The second thing you want to consider is how much support you can get from the community around it (the more popular a distribution is, the bigger the community behind it). If you are considering GNU/Linux for a company or for professional use, take into account the available professional support. So here goes my recommendations:
1. You are not a computer expert, you just want to install an OS from a CD and use it for common tasks? I'd recommend: Ubuntu, Mint, Mageia, openSUSE, PCLinuxOS... These distros are easy to install, intuitive and desktop-oriented, with a large community that can provide support when needed.
2. You like to tinker with computers and want control and flexibility? I'd recommend: Debian, Fedora, CentOS, Slackware, Arch, Gentoo... These distros are a little bit more complex to install and customise but they give you a lot of control over your system.
3. You are looking for a secure and reliable distribution that can be installed in a large number of computers and receive professional support when needed? I'd recommend: Red Hat Enterprise (RHEL), Ubuntu or SUSE.
4. You value freedom and the use of an exclusive FOSS repository? Try Trisquel, gNewSense, Blag or any of the distributions listed here.
5. You are not in one of the above categories? Take a look at this site. There you can choose a distribution based on criteria that you specify. It is a great way to discover new distributions and be amazed by the extensive choices that Free Software can give you. Here you can chose a distribution that will fit specific needs like for example: security, privacy, telephony, gaming, multimedia... and so on.
I hope this will help you understand and be aware of the options that GNU/Linux distributions offer you. The choice of a distribution is a matter of personal taste but it is often defended as a religious idea. People tend to consider one distribution to be superior and despise all others. Since there are people like that in practically all distributions, you have to follow your heart and try many of them to find the one you prefer. It is usually an intriguing and exciting adventure into the Free Software world. The important thing is to keep your mind open and have fun!