How Well Known Are the Worlds?

How well understood are the worlds, and who is there to understand them? I mean, most people only really know the world that they were in. And then you'd start sort of mapping out what the worlds are like by talking to a bunch of people about them. There i still clear that people's understanding of ... broad categories of world its still inevolving thing,. This is not like some, you know, ancient magic thing with this long history.