Debian is a widespread Linux distribution, which is used mainly on desktop machines, but numerous people have started using it lately as a web server OS too, because of the stability and protection it provides. The latter is a result of the fact that Debian is developed and supported by thousands of qualified volunteers around the globe, therefore any patch which is required to fix some issue is released in a few days. The same community can also help you with any question or problem that a user may ever have, since they can be contacted through email using mailing lists and a qualified response will be sent in around 15-20 min. The advantage here is that your queries will be responded to by the same people who have developed the Operating System itself and they have a detailed knowledge of how it works. As Debian is provided for free, you won't have to pay any license fees to use it and you can customize it as much as you want or need. This is valid for both the Operating System and the 30.000+ software packages that are readily available for it.