UNIX is a multi tasking, text-based operating system that has been adopted in modern times as the operating system of choice for web servers. It’s simple text command usage is well suited to this environment and can be accessed remotely using Telnet. The Open Group currently owns the UNIX trademark and allows it to be used on any certified UNIX based OS that complies with it’s Single UNIX Specification. Apple’s OS X is based on UNIX and so is Linux and FreeBSD.
Unix was designed to be portable (useable on multiple computing platforms), multi-tasking and multi-user in a time-sharing configuration. This makes it well suited to web server applications that make use of all of those features. When multiple people are accessing a web page via a browser, they are sharing the resources of the machine. Any operating system that is running on that machine needs to handle each user separately but at the same time. UNIX is more than likely going to be the industry standard for this application for the foreseeable future.