I knew about the regular Wikipedia, but I didn’t know about the Simple English Wikipedia until I stumbled across it at http://simple.wikipedia.org/wiki/Main_Page . As you might suspect, the Simple English Wikipedia is written in simpler language. There’s a whole set of standards set up around Basic English and the Simple English Wikipedia has a page devoted to writing Simple English articles. If you review that page you’ll get a better idea of who this Wikipedia is designed for.
This Wikipedia is a lot smaller than the main one — a little under 27,000 articles. Even though this is the Simple English Wikipedia, the concepts here are anything but simple; topics include physics (though there have apparently been some issues with the quantum physics page), engineering, several religions, and philosophy.
There’s a fairly busy SimpleTalk forum here, and a School Gateway Page for students wanting to access and edit this Wikipedia.