Quora is Yahoo!Answers done really well... it's more social and bloggy, and it's for chat and subjective questions. It's kind of like Twitter in a Q&A format with long, blog-type answers. It's full of VCs and tech journalists, unlike Yahoo!Answers which is full of teenagers asking questions about reproduction in mammals.
Stack Exchange is more a reference tool... something you use when you NEED a specific answer to an actual question that actually has an answer. We're the reference section of the library, they're an awesome salon where smart people are shootin' the shit.