These pages are deprecated (latest changes date from end 2001). More recent information may be found at http://www.coresequence.com/training.php. I leave the old pages here as they are because lots of people are still visiting them (eventhough they aren't spell-checked), and because the basics are still useful.
In order to understand the popularity of Unix, one needs to know about its history. Unix was originally developed as a multitasking system for minicomputers and mainframes. Development started at Bell Labs in 1969 througout the 1970s, with the following goals in mind:
the system had to be simple and elegant
it had to be written in a high-level language rather than in assembly code
it should allow code recycling (re-use of code)
Typical vendor systems at that time were extremely large and all written in system specific code, whereas Unix uses only a small amount of system specific code (it's called the kernel). The Unix development group worked primarily in this higher level programming language to develop the operating system on top of the kernel, thus allowing easy portability to a wide range of hardware platforms: the only piece of code that really needed to be adapted in order for the system to run on a different platform, was the kernel.
These portability and system programming issues have caused many hardware and software vendors to choose Unix in one flavour or another as an alternative to their own proprietary systems. Unix makes their applications more portable and provides vendor-independent networking, allowing users to easily interact with systems from different vendors.