There are at least three sites that can provide you with loads of information on most things mathematical:
Wikipedia. The articles there are good enough that, although I probably wouldn't cite any of them in a research paper, I would likely start there to find out where I can find out more information.
MathWorld Lots of good stuff here, too, although it's a bit more compressed, usually, than wiki.
The Springer-Verlag Encyclopedia. This is a recent find for me. I haven't used it much, but the articles I've seen are excellent.
Hope that helps.