Debian is a popular Linux release, 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 security it provides. The latter is a consequence of the fact that Debian is developed and supported by thousands of qualified volunteers worldwide, so any patch that is necessary to fix some issue is released within a couple of days. The same community will also help with any question or issue which a user may ever have, as they can be contacted via e-mail using e-mail lists and a competent response will be sent in no more than 15-20 min. The advantage here is that your queries will be replied by the same people who have designed the Operating System itself and they have a detailed knowledge of how it works. As Debian is free, you will not 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 as well as the 30.000+ software packages that are available for it.