This is most often referred to when referring to LCD screens, although subpixel antialiasing itself does not depend on an LCD. There is a technique which Microsoft calls ClearType, which exploits the three-band colour nature of LCD pixels to effectively triple the horizontal resolution. When used with subpixel antialiasing, this makes text extremely attractive, although often colour artifacts can be seen depending on the screen. Turning ClearType on on a CRT just makes everything horribly blurry produces terrible colour artifacts -- don't do it, it's not right. 

  

 ClearType has been patented by Microsoft and is only available in Windows XP, although both Freetype2 under X11 and Mac OS X use this technique as well, so the area the patent covers is at the moment unclear. Mac OS X also exploits subpixel antialiasing for rendering 'standard' antialiased text on CRTs, resulting in the far superior (or horribly blurry and ghastly, depending on your viewpoint) text quality under Mac OS X.