I assume you come to this post with experience of Microsoft Windows or Apple MacOS, since they are the only two big Operating Systems you could use and not know of GNU/Linux (as Linux is properly known) or UNIX in general.
GNU/Linux is an Operating System, like Windows or MacOS. In structure it is based on the UNIX structure, hence the two terms are kind of interchangeable. UNIX is a family of Operating Systems used for servers and other techy behind-the-scenes computer work, is old, often quite slow, and proprietary. Whic brings me to a summary of what makes GNU/Linux good:
* GNU/Linux is Free Software, meaning you can download it for free, and you are free to use it as you wish (through getting the source code you can copy, modify and redistribute the code).
* GNU/Linux is fast and stable, making it a joy to work with and very weel suited to web hosting (unlike Windows which is slow and unstable in comparison)
* GNU/Linux has a great community behind it, meaning it progresses at astonishing rates and you can continuously upgrade your system at no cost and with few worries.
The only down-side is that, comapred with Windows or MacOS, it is more difficult to install and use. Though that is changing as distributions of GNU/Linux improve.
For more info on GNU/Linux, see:
For good reviews of different GNU/Linux distributions see: