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It's part of Mac OS X's UNIX heritage. X11 is the universal windowing system for UNIX OS's (e.g. Linux, BSD, Solaris, etc.). It's very powerful, a bit abstruse, and a bit less pretty than Aqua.

Most people who don't have a UNIX background probably won't ever need to use it. There are some notable exceptions. For example, Gimp, the free photoshop alternative, is an X11 application.
 
It's part of Mac OS X's UNIX heritage. X11 is the universal windowing system for UNIX OS's (e.g. Linux, BSD, Solaris, etc.). It's very powerful, a bit abstruse, and a bit less pretty than Aqua.

Most people who don't have a UNIX background probably won't ever need to use it. ...
Although the X Windowing System is ubiquitous on UNIX and UNIX-like systems, it is by no means restricted to such systems. X Windows has long been available for the MacOS, Windows, DEC VMS, and other operating systems.
 
Openoffice.org is also an X11 app. So is X11 essentially a filter that allows you to run Unix apps on OS X?

It's not a filter. It's a windowing system. In Mac OS X, it's an application that is used for X11 applications. Think of it as Quartz for Unix/Unix-like OS's. They both provide the same function. For Unix/Unix-like OS's, X11 works the same as it does on Mac, except it's initiated at startup and there is no X11 console like there is in Mac OS X (there's still a console though).
 
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