Debian is an Operating System that uses the Linux kernel. Although it is usually set up 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 won't need to be restarted for months. Debian is also regarded as being one of the most risk-free Operating Systems in existence, because security patches are released within days after a problem is discovered. This is possible since thousands of well-experienced developers are part of the Debian project on a voluntary basis and not only do they work on the OS itself, but they also provide 24/7 support via special e-mail lists and respond in 20 min to any question. Thus, 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 several Linux distributions with the highest number of packages - more than 30,000. Since the Operating System is completely free to use, you will be able to modify both the core as well as all the packages in the way you see fit. In addition, you won't have to pay any kind of license fees.