The UNIX operating system market is extremely diverse, with a myriad of choices for the Intel platform. As a result, it's necessary to use broad terms when referring to its member constituents. In many ways UNIX is the "old pro" of operating systems. Having originated in the 1960s on minicomputers and mainframes, its strengths are a fundamentally sound architecture, excellent portability (as evidenced by its presence on nearly every CPU platform around), and inherent networking capabilities.
Perhaps the biggest attraction to UNIX is the sheer number of implementations on the market. There is literally a flavor of UNIX available for every major CPU architecture, from high-end workstations to a simple Intel 486 PC. When a new CPU comes out, the first priority for the vendor is often to get a UNIX variant up and running as soon as possible. This, in turn, translates into a broad selection of potential hardware platforms, including most non-Intel RISC architectures, for the UNIX user.
Unfortunately, UNIX's proliferation has also been a curse in that it has lead to few viable, shrink-wrapped applications. Though the operating system itself is highly portable from CPU to CPU, there are many cases where a UNIX application written for one flavor of the OS will fail to run properly on another flavor -- even when the underlying hardware is identical.
The lack of true standardization in the UNIX API arena has relegated this powerful, if somewhat dated, operating system to the role of database server or high-end workstation environment. While these are not typical desktop PC operating system roles, they do highlight UNIX's other main strength: networking support. UNIX is inherently networkable, with extensive support for TCP/IP and distributed file systems like NFS from Sun Microsystems.