Unix
Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson and Dennis Ritchie. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit organizations.
The present owner of the trademark UNIX® is The Open Group, an industry standards consortium. Only systems fully compliant with and certified to the Single UNIX Specification qualify as "UNIX®" (others are called "Unix system-like" or "Unix-like"). Similarly, POSIX defines a standard set of system calls, utilities and standard library functions for Unix-like systems.
During the late 1970s and early 1980s, Unix's influence in academic circles led to large-scale adoption (particularly of the BSD variant, originating from the University of California, Berkeley) of Unix by commercial startups, notably Sun Microsystems. Today, in addition to certified Unix systems, Unix-like operating systems such as Linux, Mac OS X and BSD derivatives are commonly encountered.
The philosophy of Unix
Although there is quite a variety among Unix systems, one common theme is the so-called "small tools that do their job very well." That is, a Unix system has many programs, each of which specializes in a small task. The user can combine these tools (via scripting or piping) to accomplish higher level goals. Although this may make some tasks more difficult than common graphical user interfaces, it allows the user to perform complicated tasks that were not explicitly allowed for by the interface's designer.
For instance, suppose the user wanted to create an archive of all files which reference his vacation created between two and three month ago. Under Unix, this could be accomplished as a combination of tar (the archiving application), find (a file search application) and grep (a file pattern matching application). In a single command:
tar czf vacation.tar.gz `find . -ctime +60 -ctime -90 -exec grep -il vacation {} \;`
In contrast, a user in a graphical user would need to use the search application to find such files, wait for the search to complete, and then use an archiving application to create the archive.