Debian is a popular Linux distribution, that is used primarily on desktop computers, but lots of people have started using it lately as a web server OS too, due to the stability and protection it provides. The latter is a direct result the fact that Debian is developed and supported by countless qualified volunteers worldwide, therefore any patch which is necessary to fix a certain issue is released in a couple of days. The same community can also help with any question or problem that a user may ever encounter, since they can be contacted via email using mailing lists and a competent response will be sent in no more than 15-20 min. The advantage here is that all your questions will be responded to by the same people who have created the Operating System itself and they have a detailed knowledge of how it works. As Debian is absolutely free, you will not need to pay any kind of 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.