Debian is a widespread Linux distribution, which is used primarily on personal computers, still a lot of people have begun using it lately as a web server OS as well, because of the stability and security it offers. The latter is a consequence of the fact that Debian is developed and supported by a large number of qualified volunteers worldwide, which means that any patch that's necessary to fix a particular issue is released in a matter of days. The very same community will also help with any question or issue that a user may ever have, since they can be contacted via email using e-mail lists and a competent response will be sent in no more than 15-20 min. The advantage here is that your questions will be answered by the same people who have developed the Operating System itself and they have a detailed knowledge of how it works. As Debian is free, you will not have 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 that are readily available for it.