X Window System
Abbreviated as X or X11, the X Window System is a graphics windows management system that began development at MIT (Massachusetts Institute of Technology) in 1984 for Unix workstations. The X Window System is a system process that handles the displaying (drawing) of windows, frames, taskbar, title bars, resizing windows, and moving windows. In fact, most UNIX graphical interfaces are based on the X Window System.
Tip: To get into the X Window System from the command line, type startx. Computers with X Window System installed will automatically start or prompt the user with the availability when the computer first boots.