Debian is an Operating System which uses the Linux kernel. Though it is usually set up on desktop machines, it is being used on a growing number of web servers as well. The reason is that it's very stable, so a server running it will not need to be restarted for months. Debian is also regarded as being one of the most risk-free Operating Systems to be found, since security patches are released within days after a problem is found. This is possible because thousands of experienced developers are employed by the Debian project on a voluntary basis and not only do they work on the OS itself, in addition, they provide 24/7 support via special e-mail lists and respond in 20 min to any question. Thus, you will get top-notch 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 distributions with the highest number of packages - over 30,000. As the Operating System is free to use, you can modify both the core as well as all of the packages in any way you see fit. Also, you won't have to pay any license fees.