Debian is an Operating System that uses the Linux kernel. Though it is usually installed on desktop computers, it is being used on more and more web servers too. 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 secure Operating Systems nowadays, because security patches are released within days after a problem is found. This is possible because thousands of skilled developers are working for the Debian project on a voluntary basis and not only do they work on the OS itself, in addition, they provide 24/7 support by using special mailing lists and reply in 20 min to any question. Thus, you will have an outstanding level of 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 and any of the packages in the way you see fit. In addition, you won't be required to pay any license fees.