Debian is an Operating System which uses the Linux kernel. Though it is usually set up on desktops, it's being used on more and more web servers as well. The reason is that it is very stable, so a server running it won't need to be restarted for months. Debian is also considered to be among the most secure Operating Systems nowadays, since security patches are released within days after an issue is discovered. This is possible because thousands of experienced developers work 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 e-mail lists and reply in 20 min 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 several Linux releases with the highest number of packages - over 30,000. Since the Operating System costs nothing to use, you'll be able to modify both the core and all of the packages in the way you see fit. Furthermore, you won't be required to pay any license fees.