A number of other advanced operating systems are on the rise in the PC world. Specifically, IBM's OS/2 and Microsoft's Windows NT are becoming very popular as more users move away from MS-DOS.
What makes UNIX so important? Not only is it the most popular operating system for multiuser machines, it is also the foundation for the majority of the free software world. If you have access to the Internet, nearly all of the free software available there is written specifically for UNIX systems. (The Internet itself is largely UNIX-based.)
There are many implementations of UNIX, from many vendors, and no single organization is responsible for distribution. There is a large push in the UNIX community for standardization in the form of open systems, but no single corporation controls this design. Hence, any vendor (or, as it turns out, any hacker) may implement these standards in an implementation of UNIX.
OS/2 and Windows NT, on the other hand, are proprietary systems. The interface and design are controlled by a single corporation, and only that corporation may implement that design. (Don't expect to see a free version of OS/2 anytime in the near future.) In one sense, this kind of organization is beneficial: it sets a strict standard for the programming and user interface unlike that found even in the open systems community. OS/2 is OS/2 wherever you go---the same holds for Windows NT.
However, the UNIX interface is constantly developing and changing. Several organizations are attempting to standardize the programming model, but the task is very difficult. Linux, in particular, is mostly compliant with the POSIX.1 standard for the UNIX programming interface. As time goes on, it is expected that the system will adhere to other such standards, but standardization is not the primary issue in the Linux development community.