Debian is a well-known Linux distribution, which is used mainly on personal computers, still many people have begun using it lately as a web server OS too, due to the stability and security it provides. The latter is a consequence of the fact that Debian is developed and supported by a large number of qualified volunteers all around the world, so any patch which is necessary to fix some issue is released within a few days. The same community will also help you with any question or issue that a user may ever have, due to the fact that they can be contacted via e-mail using e-mail lists and a competent response will be sent in around 15-20 min. The advantage here is that all your questions will be replied by the same people who have created the Operating System itself and they have a detailed knowledge of how it works. As Debian is free, you won't need to pay any kind of license fees to use it and you'll be able to customize it as much as you want or need. This is valid for the Operating System and the 30.000+ software packages which are readily available for it.