Ask Dr. Math is a service of Swarthmore University's Math Forum. Over 300 Swarthmore math graduate students are currently volunteering to recreationally answer K-12 students' questions about math.
They cover every problem imaginable to young curious minds in their archives. It's usually a better resource for learning than Eric Weisstein's World of Mathematics as the responses are more an explanation rather than a definition.
The Ask Dr. Math site can be found at http://askdrmath.com/dr.math.