The worlds in science fiction are almost invariably completely made up. Of course, they're frequently based on current human points of view and habits. What else do we know so well?
But worldbuilding is something that varies widely between authors.
As I see it, there are four extremes: the shallow ocean, the shallow pond, the deep pond, and the deep ocean.
There are the authors who show a large swath of their world, but the differences from our own are few, or perhaps the implications of the differences have not been fully worked out. Why would you write about a world with working nanotech and still have factories in it?
There are the authors who show only a small amount of the world, and have likewise not developed the implications or have few differences from our current culture. Sometimes the character's story is so central to the story that the world doesn't have to be very different from our own, or just doesn't seem important to the author.
There are authors who show only a small amount of the world, but what is shown is consistent with a very highly developed, if hidden, background. Any differences between our culture and the story culture have had their implications worked through to varying depths and integrated into a whole.
Finally, there are authors who show large swaths of a world worked out in excruciating detail. Actually, this takes two forms: epic, where the author simply has a helluvalot to show as part of the story (think Lord of the Rings); and 'look, I had to read about all this stuff in order to write this book, so you get to read it too', where the author puts in all kinds of information irrelevant to the immediate plot but necessary to the world (think *shudder* Jean M. Auel's Clan of the Cave Bear series - specifically the later books).
What type of worldbuilding do you do? (These are extremes, so most people will fall in between.) What type of worldbuilding do you prefer to read?