Debian is an Operating System which uses the Linux kernel. Though it is usually set up on desktop machines, it is being used on an increasing number of web servers too. 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 among the most safe Operating Systems to choose from, because security patches are released within days after a problem 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, but they also provide 24/7 support through special e-mail lists and reply within 20 min to any kind of question. Thus, you will obtain top-notch 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 - over 30,000. Since the Operating System is free to use, you're able to modify both the core as well as any of the packages in any way you see fit. Furthermore, you won't be required to pay any license fees.