Sarah Hoyt has a wonderful piece on that over on her blog According to Hoyt. The key point is that when you write a story you’ve got this picture of the world and the people in it (even if they’re not “people” but walking cabbages or what have you). Your reader, however, doesn’t. They come into your world knowing nothing about it or about your characters and the only things they know are what you tell them and what you show them.
Picking the right details to show, and when to show them, so as to create the picture you want to create, while at the same time not bogging down the story in excess detail, is a constant balancing act. Ideal is providing those relevant details in ways that push the story along. Heinlein was a master of this, better in some cases than others but very good at the telling little detail that establishes setting. The classic example is “the door dilated” from Beyond this Horizon. Everyone knows what a door is. That it “dilated” rather than opened immediately told us that we were in a world different from the normal one.
Another excellent example is the Heinlein’s novel Friday. The opening scenes just drip with details that set the scene all without bogging down a remarkably fast-paced opening.
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