Unix is the oldest and one of the most widely used operating systems for the web hosting platform. It is the basis of the internet and most web servers originally ran on a version of Unix. Unix is know as a multitasking operating system which means that it can run many applications at the same time without them affecting each other. This is very important when you consider that hundreds of websites, running thousands of CGI scripts, all receiving thousands of hits could be running on one web server. Unix is usually more reliable than Windows hosting and more companies are on UNIX than windows simply because it is faster and open source (meaning it is completely free unlike Windows which has hefty license fees).