The darker a point appears on the map, the more times it has been viewed.
Each square represents one unit of imagery, called a "tile". When the program starts, it shows tiles at zoom level 11, which has tiles at a resolution of 74 meters-per-pixel. At the closest in, Virtual Earth has tiles at zoom level 19, 0.3 meters-per-pixel. You can look at higher- or lower-resolution points with the "select data level" indicator at the top.
A sample of imagery and a relative sense of what was in a given spot is available at each scale by right-clicking on the map. The "locator map" shows imagery at the specific point clicked.
This data is based on a sample of tile logs on servers from January through July of 2006; it is not live. It combines server hits from road, aerial, and hybrid imagery in one view.