I have read vague references to mathoverflow being integrated with stackexchange. Was mathoverflow a separate company or something? Whats the story?
The first thing was StackOverflow, for programmers to help each other. Perhaps some offshoots for other topics had already happened, but MO was set up on SE 1.0 and running by September 2009. This place, MSE, started roughly a year later, for all levels of mathematics.
MO is now an SE 2.0 site, but with some legal documentation that allows MO a good deal of self government. The most important difference is that MO is by and for mathematics researchers, mostly graduate students and professors. Most people use their real names. There are enough people active on both sites that lots of very difficult questions asked on MSE are handled here.
Well, probably enough. I'm going to sleep now. I will check this again i the morning, but I imagine you will have lots of information from others by the time I wake up.