The page, by default, searches HTML resources, including the W3C specs and Microsoft's MSDN materials. Those and the w3schools website show up often in the other languages available - CSS, XSL, XML, and XPath. The PHP manual that's referenced is just pulled from the PHP.net site, but it's still nice to have it all right there.
When searching, a "suggeted terms" box pops up to help you narrow down the search faster. It searches not only the beginnings of the words for your term, but it looks inside them as well, matching anywhere.
It would be nice if the site had some kind of bookmarking system built into it - then you could keep a reference and just flip back and forth between the pages with a single click of the mouse.