Debian is an Operating System that uses the Linux kernel. Though it is usually set up on personal computers, it's being used on a growing number of web servers as well. The reason is that it's really stable, so a server running it will not have to be restarted for months. Debian is also regarded as being one of the most protected Operating Systems you can get, due to the fact that security patches are released within days after a problem is discovered. This is possible since thousands of qualified developers are employed by the Debian project on a voluntary basis and not only do they work on the OS itself, they also provide 24/7 support through special mailing lists and reply in 20 minutes to any kind of question. Thus, you will get top-level support directly from the developers who know their own platform in depth, and not from a third-party support team. Debian is also one of the Linux releases with the highest number of packages - more than 30,000. Since the Operating System costs nothing to use, you will be able to modify both the core as well as all the packages in any way you see fit. Also, you will not have to pay any license fees.