A/UX (short for Apple Unix) was Apple Inc's first Unix-like operating system. It only ran on certain 68k Macintoshes (according to Wikipedia, only 13 models). Much like its modern-day successor, Mac OS X, its GUI interface was based on the standard Mac OS look-and-feel (looking almost identical to then-current System 7). Unlike the Mac OS of its time, A/UX included a command line interface, along with a GUI program to simplify selecting various terminal programs' options and switches.
In addition to its own programs, A/UX was also able to run standard MacOS programs via a compatibility layer (similar to modern-day Wine).