Debian is a Linux distribution, which is widely used for desktop machines, yet it has been becoming more popular as a server OS over the last couple of years too. It's very stable and safe, because fixes are released within a couple of days after an issue is discovered. Debian has a large support community, so when you have any kind of questions or problems with the OS or with any of the 30,000+ packages it comes with, you'll be able to contact the developers without delay via e-mail using special mailing lists and receive a reply within 15-20 min. The Debian community offers the top level of support possible, as all of your questions will be handled by the very same people who made Debian in the first place, so they have an exhaustive knowledge of how their own system works. Because the Operating System is absolutely free to use and distribute, no license costs will be included with your web hosting fees and you'll be able to change the code files and any additional software that you install with no restrictions, to optimize the software environment for your websites. Furthermore, Debian performs very well on any hardware, because it's light and quick and doesn't need a lot of memory.