Unix is an operating system, or OS. Just like WindowsXP or Vista or Linux is an operating system on your own computer, Unix is an OS commonly used on web hosting server computers. Unix has variations, amongst which is Linux. They are essentially very similar and you will find that the vast majority of hosting servers around the world run on Unix (or Linux). Some web servers have a Windows or other OS and generally these are somewhat more expensive. Unix is so common because of its relative simplicity and robust structure.