Debian is a well-known Linux distribution, which is used primarily on desktop machines, however lots of people have started using it lately as a web server Operating System as well, due to the stability and protection it offers. The latter is a direct result the fact that Debian is developed and supported by a large number of qualified volunteers all around the world, so that any patch which is necessary to fix some issue is released in a couple of days. The same community will also help you with any question or trouble which a user may ever experience, because they can be contacted via email using mailing lists and a competent response will be sent in around 15-20 min. The advantage here is that all your questions will be responded to by the same people who have made 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 the Operating System and the 30.000+ software packages which are available for it.