In 1988, an organization called the MIT (Massachusetts Institute of Technology) X Consortium was formed to promote and develop a vendor-neutral windowing system called the X Window System. (It was called "X" because it was a follow-on to a window system called "W" that was developed at Stanford University.) The organization eventually moved away from MIT; now it's called the X Consortium.
A window system is a way of dividing up the large screen of a workstation into multiple virtual terminals, or windows. Each window can contain a separate application program. While the "big win" is to have applications with point-and-click mouse-driven user interfaces, one of the most common applications at this point is still a simple terminal emulator (xterm). X thus allows a workstation to display multiple simultaneous terminal sessions. This makes many of the standard UNIX multi-tasking features such as job control less important, since programs can all be running in the foreground in separate windows. [The X Window System has also brought about a display terminal called an X terminal. An X terminal gives users access to X without buying a complete workstation. -JP ]
Not everyone has X, though, and it's a whole separate can of worms, so we don't cover X or any other window system in this book. If you like this book, look for X User Tools from O'Reilly & Associates.