The line can be blurry sometimes, but it's roughly the difference between research and teaching. MathOverflow is meant to help mathematics researchers. At its b

The line can be blurry sometimes, but it's roughly the difference between research and teaching. MathOverflow is meant to help mathematics researchers. At its best, it's like being able to walk over to a colleague's room in a prestigious math department to ask for insight. Math.SE, on the other hand, is for "people studying math at any level and professionals in related fields" and should be used for questions that didn't originate in research, such as hard textbook exercises. Both are super interesting, and even mathematicians will have plenty of questions that more suitable for Math.SE than MathOverflow.