There's a complete list on http://stackexchange.com/sites; there are actually way more than 41 sites in the SE network.

It's much more focused on: 1) specific technical questions; that 2) plausibly have a good answer. Quora is a more general Q&A site, where you can ask questions on pretty much any subject, and can ask open-ended/discussion type questions as well.

There appears to be a trend of sites creating micro communities. Reddit have done something similar. Quora does it.

The SE guys do a podcast which gives an insight into their business.