Aerial maps, as their name would suggest, are photo maps taken from up in the air. Many times they can be satellite maps. By looking at aerial maps over time, one can see changes that took place in an area. Unfortunately, aerial maps have a much shorter history than other maps as the history of aviation and photography is much shorter than that of cartography or the making of maps.
One great source of aerial maps is the Microsoft Terra Server that was created in partnership with the USGS. It is one of the world's largest servers containing digitized images. Many of the images were declassified spy satellite pictures from the Cold War!
To view these maps, go to the following link: