Debian is a widespread Linux distribution, that is used mainly on personal computers, but many people have begun using it lately as a web server OS too, because of the stability and security it provides. The latter is a direct result the fact that Debian is developed and supported by thousands of qualified volunteers internationally, so that any patch which is required to fix some issue is released in a matter of days. The same community will also help you with any question or trouble that a user may ever encounter, since they can be contacted via e-mail using mailing lists and a qualified response will be sent in no more than 15-20 min. The advantage here is that your questions will be responded to by the same people who have created the Operating System itself and they have an in-depth 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 as well as the 30.000+ software packages which are readily available for it.