Lighter areas contain newer construction, while darker areas are older. What stands out immediately is the difference between the Sunbelt--areas in the Southeast and Southwest where growth has been booming--and the Rustbelt--an arc stretching around the Great Lakes from North Dakota to New York. Nebraska and Kansas haven't seen much in the way of new construction either. This tracks pretty well with the overall population change in these areas. Over the past 10 years people have migrated to the Sunbelt and away from the Rustbelt.
Urban areas stand out as dark clusters of old housing stock surrounded by coronas of newer development. To understand why housing in the suburbs is so much cheaper, you need look no further: more construction equals more supply. Here's Atlanta, for instance:
And here's Chicago:
Note especially that the Atlanta map is much lighter overall. There's simply more new construction happening there and in many other Southern cities compared to Northern ones, even in the suburbs. Look at New York in the main map, for instance. There's not much in the way of new construction happening even in the suburbs surrounding the city. Same holds true in San Francisco, so it's no surprise that housing prices in those two cities are among the most expensive in the nation.
Finally, for local readers, here's what D.C. and Baltimore look like. For everyone else, take a look at the big map and let me know what you see in your neck of the woods.