Debian is an Operating System which uses the Linux kernel. Although it is usually set up on personal computers, 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 won't need to be restarted for months. Debian is also considered to be among the most protected Operating Systems nowadays, since security patches are released within days after an issue is found. This is possible because thousands of well-trained developers are part of the Debian project on a voluntary basis and not only do they work on the OS itself, they also provide 24/7 support via special mailing lists and respond within 20 min to any kind of question. In this light, you will obtain top-level 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 releases with the highest number of packages - more than 30,000. Since the Operating System costs nothing to use, you can modify both the core as well as all the packages in the way you see fit. Furthermore, you will not need to pay any kind of license fees.