It's a pretty good website. It explains a lot of the areas in math. The 'Schools' page, for example. After clicking under the thing about the encyclopedia, there's a small list - Each reading 'What is ______?'
The rest of the site is pretty good, too. It has some pretty good tangrams and math puzzles (not as many as MathsIsFun, though!), and it can keep you entertained for quite a while. Also, this website gives the basic explanations for each of the subjects there. For example, under Trigonometry, it explains a lot about using sine and cosine. Some of the better pages can be surprisingly difficult to find, though, which is a little bit of a disappointment. For example, this little hidden page here:
It was hard to find, but it was worth it in the end. I'd say this is quite a valuable resource to some students.