Debian is an Operating System which uses the Linux kernel. Although it is usually installed on desktop machines, it's being used on more and more 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 considered to be one of the most safe Operating Systems you can get, since security patches are released within days after an issue is found. This is possible since thousands of qualified developers are working for 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 min to any question. Thus, you will obtain 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. As the Operating System costs nothing to use, you're able to modify both the core and all the packages in any way you see fit. Also, you won't have to pay any kind of license fees.