In their own words ...

"PlanetMath was conceived by Nathan Egge ("unlord" on PlanetMath) and Aaron Krowne ("akrowne" on PlanetMath-- yours truly) as a project to fill the void for a central repository for mathematical knowledge on the web, with a pedagogical slant. But we realized that we could do it in a way that would make it a community, built by the users, and that the users would be just as valuable as the content itself. There are now many grassroots-content sites like this (Wikipedia, Citeulike, Slashdot, Fark...), and PlanetMath represents a unique kind of hybrid of MathWorld, Wikipedia, and Slashdot concepts. We think that the end result has proven very useful to many people who are interested in learning math, who are already students studying math, or who make their living in mathematical professions."

In other words, it's a math reference site that is highly collaborative in nature. It has been around for a few years now and has accummulated a fairly deep and broad collection of content.

T4.