One thing that confuses people coming to Linux from other Operating Systems is that are so many different "Linuxes" [or should that be Lini?]. In my own blog I often mention three different distributions Ubuntu, Fedora and Mandriva (formally Mandrake). I use another at work RedHat, and there are lots more different distributions.
The fact is there is really just the one Linux, but lots of distributions. Linux actually only refers to the kernel of the operating system, then there are some essential programs which are classed as part of the operating system. These are generally taken from gnu, so really the operating system bit is GNU/Linux.
The thing that most people consider to be Linux is called a distribution. Rather than just being an operating system it is a whole collection of different applications, including a web server, a desktop environment, office suite etc. This is often referred to as “Linux”, but it incorporates far more than just an operating system.
There are a number of differences between the different distributions depending upon your needs. No one distribution is better than the others, it all depends on what you want to do with your system and what support you expect.
Support and Cost
Some distributions are completely free, but then rely on community support or charge for support whereas others charge a license fee per installation. Effectively you are not really paying for the software at all, just for someone to help you out if you get stuck / find a problem.
If you are running Debian then you have only community support, if you choose RedHat Enterprise Linux then you end up paying for commercial support in the license, or if you use Ubuntu then you can have either free community support, or pay for commercial support from Canonical (the company that funds Ubuntu development).
There are hundreds of free programs available, and someone has to decide which software goes into the distribution and which doesn’t. Different people have different needs. Some distributions are tailored towards the desktop, including a desktop environment, office suite and photo editor, others are geared towards the server environment including a web server, database and samba (windows networking) server
Some others try to do all of these, or some try and put as many software programs as possible (up to 7 CDs), or as few as possible (single floppy disk). Debian in it’s 3.0 release includes everything they could find on 7 CDs, whereas Ubuntu only provides a single CD with cherry picked applications. Fedora, RedHat and Mandriva come somewhere between the two extremes. All these have a means of installing any applications that you want that were not provided on the install disk.
More specific than having different roles there are often different applications that do the same thing. For example there are different desktop environments (KDE and Gnome), the choice of which is often personal preference. There are different programs for playing music, or burning to CD and even different web servers (although Apache is by far the most common).
Different Versions of components
As software develops new version are released. Some distributions try and include the latest releases of all software (called cutting edge), whereas others try and use older versions that have been proved reliable and had more bugs fixed (sometimes called production ready). Some distributions fluctuate between releases with some classes as “production ready” releases and others “cutting edge”, and some try and position themselves between the two. To look at Fedora and RedHat Enterprise Linux (both created by RedHat) they use Fedora to add new cutting edge applications to, but only put tried and tested versions into the commercial RedHat Enterprise Linux releases.
Different Package Management
The way that software is installed is handled differently. The two main ways being RPM (based on a RedHat solution) and Dpkg (based on Debian). Both have their pros and cons and supporters.
When it comes down to it, it is not about which distribution is better, but about which one fits your requirements. More importantly it’s about choice and the freedom to do what you want with the operating system.
I have also added this as a Penguin Tutor Tutorial Document – What is a Linux Distribution?.