So let's say you write a web app in Rails or Django. Of course all sorts of magic happens behind the scenes, but ultimately the whole thing boils down to a function, ( def handle_request(request) { return ... } ); which is given the request and returns a response. If you have to wait for something (the database or whatever) that function will have to sit around waiting for it (e.g. "blocking") before returning the answer.

In Node.js, despite however much magic of a framework you have, it utlimately boils down to a function that gets a request and a response proxy ( function process_request(request, response) { } ). The function can do whatever it wants with this response object, by sending data right away, stashing it somewhere to be picked up later, having it send data in response to an event in the future, whatever. Then the function returns, and gets out of the way, and other stuff runs. But the response object is still sitting around, and whenever you tell it to do something, you can send some (or all) of a response back.