UNIX is currently a trademark of X/Open Ltd (*). UNIX was developed
at AT&T Bell Laboratories in the late 1960's and has become the most
portable operating system ever developed.  This portability means that
UNIX (or Unix-like) operating systems can run on nearly any computer
that has been developed since 1975.  Unix-like operating systems run on
computers from the Intel 8086 through the gamut to computers like the
Cray X-MP and the Connection Machine 5.  Programs written for one
Unix-like system can be moved (fairly easily, if written with portability
in mind) from one Unix system to another with some minor editing and
re-compiling of the source code on the new machine.

The history of Unix-like operating systems is quite complicated, and
there are several versions of Unix-like operating systems in existence.
The two major divisions of these are AT&T UNIX systems, and BSD Unix
systems.  There are lots of others, but these two versions cover most of
the territory in terms of portability issues.  Truly portable Unix
software packages can examine the software environment and automatically
configure themselves when the user compiles the package.

Unix operating systems are flexible and portable for many reasons, and a
detailed examination of that issue is not relevant here.  For more
information see the Unix-FAQ posted regularly to comp.unix.questions.

(*) Just who *is* the actual holder of the TradeMark on UNIX is
something that can change with bewildering rapidity.  At various times
it has been the "offical" property of: AT&T, Bell Laboratories, Western
Electric and other groups.  These names, of course, are also registered
TradeMarks of their respective owners. :-)