I've just spotted this site on the blogs. At least that's where I think I saw it first.
How is this site different from math.stackexchange.com?
This site is mainly for professional research mathematicians, that is professor of mathematics at universities, advanced graduated students in mathematics (students working towards a doctoral degree in mathematics and close to completing their studies), and alike.
The primary goal of the site is to have a place to allow them to ask and answer research level mathematical questions, the sorts of questions they come across when writing or reading articles or graduate level books. [This sentence is essentially how the official description of the site started when it was not yet part of the network but existed as a standalone site, which it did for about three and a half years; see an answer on Frequently asked questions for MathOverflow for a fuller version.]
Certain other types of questions that are not mathematical questions, but for example questions specific to the work-life of a research mathematician, sometimes get also asked on the site. Whether or not they are acceptable is a matter of continuing debate and there is no actual consensus on this matter. The reality is they do get asked, some of them stay open, some of them get closed. The official suggestion is not to ask such questions before being familiar with the site. (See the question mentioned above.)
It should however be stressed that not everbody on the site falls in the above mentioned categories and there are certainly no formal requirements to participate in the site. However, for somebody not having a mathematical knowledge quite close to that of somebody having a doctoral degree in mathematics (think of at least five years of studying mathematics at the university-level) this site is likely not a good fit (at least as an active participant).
By contrast, math.SE, the site you mention, which counts among its users also many research mathematicians (in fact there is a considerable overlap of the respective user-bases) welcomes mathematical questions at all levels. Numerous questions from this site are transferred to math.SE, as they are perceived as not being at a suffciently advanced level for this site.