Debian is an OS that uses the Linux kernel. Though it is usually installed on desktops, it's being used on an increasing number of web servers as well. The reason is that it is very stable, so a server running it won't have to be restarted for months. Debian is also regarded as being one of the most protected Operating Systems out there, due to the fact that security patches are released within days after an issue is discovered. This is possible because thousands of well-experienced developers work for the Debian project on a voluntary basis and not only do they work on the OS itself, but they also provide 24/7 support by using special mailing lists and reply in twenty minutes to any kind of question. Thus, you'll have top-notch support directly from the developers who know their own platform in detail, and not from a third-party support team. Debian is also one of the Linux distributions with the highest number of packages - over 30,000. As the Operating System is absolutely free to use, you will be able to modify both the core and all of the packages in any way you see fit. Furthermore, you won't have to pay any license fees.