NOTICE: All Unix questions must be related to programming; those that aren't will be closed. Use this tag only if your question relates to programming using Unix APIs or Unix-specific behavior, not just because you happen to run your code on Unix. General software issues should be directed to Unix & Linux Stack Exchange or to Super User.
Tag usage
The tag unix can be used for Unix system programming related problems. The tag can also contain programming questions about using the Unix platform. For non-programming Unix usage questions, visit the Unix & Linux Stack Exchange site.
Background
Unix was developed as an in-house operating system for AT&T, but in the 1980s became both a prime academic operating system (with U.C. Berkeley's version, called BSD, being the reference platform for development of what would become the Internet) and a commercial success in the form of AT&T's System V, Microsoft/SCO's XENIX (PCs) and various workstation versions from Sun, Silicon Graphics, and others.
In the 1990s, Sun's Solaris and the free Unix clone Linux would rise in popularity. Linux is largely Unix-compatible but lacks the trademark. Currently, Unix is commonly found on server platforms; the primary desktop variant is Mac OS X, based on BSD.
Apart from its command-line interface, most "Unices" support the standardized X Window System for GUIs. (So does Mac OS X, but its primary GUI is Apple's proprietary Quartz.)
The various Unix implementation (and to a lesser extent, clones such as Linux) are unified in a standard called POSIX. C has been its primary programming language since the 1970s, but many other languages are available.
