Debian is an OS that uses the Linux kernel. Although it is usually installed on personal computers, it's being used on a growing number of web servers too. The reason is that it's really stable, so a server running it won't have to be restarted for months. Debian is also considered to be among the most risk-free Operating Systems in existence, as security patches are released within days after an issue is discovered. This is possible since thousands of skilled developers are employed by 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 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 the Linux distributions with the highest number of packages - more than 30,000. Since the Operating System is free to use, you're able to modify both the core and all of the packages in any way you see fit. Also, you won't need to pay any license fees.