Atlas is a very popular addon by Dan Gilbert, the author of Hacking World of Warcraft (book). It allows viewing of maps of areas (such as instances and battlegrounds) that normally don't show in the world map window. It also includes maps of flight paths and boat and zeppelin routes.
Most of the maps in Atlas are constructed from views seen on the minimap. Atlas maps usually include numbers and letters on the map with a key to the right. These numbers and letters indicate bosses, key locations (items, profession-related or quest-related), and entrances.