This IS X11. This is where Apple contributes to and sources what they provide with Mac OS X as X11. The XQuartz project is typically well ahead of what Apple provide.
This is GEEKWARE. It requires knowledge of using the CLI found in the Terminal app for full capability. If you don't know what X11 is, if you don't know what MacPorts are or what Fink/Porticus/PortAuthority/Pallet are, then don't download XQuartz. Don't install it. Don't write a one star review out of ignorance. Learn about X11 and XQuartz first.
I use X11 apps and features frequently. I love this stuff. But I am a geek and I have taken the time to learn how to use this. I also have unfathomably patience setting up this geek level stuff. It takes unfathomable patience to use X11 because it is designed by geeks for geeks, not for average or even intermediate users. It is in fact a big, frustrating PITA to use X11 successfully! But the rewards are a lot of fun.
ALSO NOTE: Apple are no longer going to provide X11 with Mac OS X! It come with the Mac OS X 10.8 Mountain Lion installer. INSTEAD Apple are going to direct Mac users to download exactly what we have here: XQuartz, the source project for X11. So if you like X11, download the latest stable XQuartz and get to work with it!
BUT! Be sure to read ALL the documentation at the source website BEFORE you download or install new versions. Because this is geekware, IT HAS BUGS. Some bugs may mean that it won't install on or work with your version of Mac OS X. Please do your homework so you're ready for the XQuartz's challenges.