The X Window System
The X Window System, X or X11 (but not "X Windows", due to trademarks owned by Microsoft) is the portion of a Linux or BSD distribution which manages the screen, keyboard, and mouse, mediating requests for these from other user applications, referred to as "X clients". MacOS/X uses a different display server technology.
Older versions of it were typically XFree86, which has transitioned into X.org; the current versions is X11R7.x which are distributed as standard in virtually all current Linux distributions.
Which one you will find depends on your operating system; most of them will work, as long as they provide (or support) a 'video driver' for your VGA card which permits you to use Xv -- the extension which makes it possible to display video overlay on your monitor without creating a high load on your main CPU.