According to Wiki, there are over 300 different Linux distributions. Some of the more well known are Red Hat Enterprise Linux, Debian, Suse, Ubuntu, Fedora, Centos, FreeBSD etc.
So here is the famous question which one to use? I was faced with the same question and could not find a definitive answer. After some hours Googling, the result.
The simplest answer is from LinuxQuestions.org
At the end of the day, the difference is on emphasis (stability for CentOS vs. hobbyist more bleeding edge stuff for FC). Ultimately, either would do fine for a home desktop.
From ReallyLinux.com on choosing Linux.
For example, I see a lot of people using Fedora as a production server – this is perfectly fine if you plan to upgrade to the latest releases every 6-9 months. However, if you don’t want to upgrade a production box that often (and in most enterprise environments, this would be unacceptable), you need to look for a longer life distribution. Redhat Enterpise Linux (RHEL)/CentOS, Debian or Ubuntu LTS editions are some good choices, but there are others around too.
If you continue running a distribution after it’s support life has ended, you risk opening yourself to security flaws – and the last thing anyone wants is your server increasing the production of spam!
A more comprehensive answer can be found at Wiki’s comparison of Linux distributions.