Map
Maps are used to help Altaïr and Ezio move about the cities, and plan assassinations or investigations. They act as a compromise and visualization of Altaïr's and Ezio's memories of the city layout, as well as give a sense of direction for the player. With every View Point you have successfully climbed and synchronized with, more areas will appear on your map. The map also marks certain key areas that can be recognized by a unique symbol.
In Assassin's Creed, the symbols indicate the Assassin's Bureau, investigation missions (Eavesdropping, Interrogation, Pickpocketing and Informants), exits to the Kingdom, and the locations of citizens you can save, or have saved from soldiers.
In Assassin's Creed II, symbols mark key memories, side memories (such as Races or Courier Assignments), stores (Art Merchants, Doctors, Blacksmiths, Tailors, and Travel Stations), treasure chests and Assassin's Tombs. The Map's frame color also indicates your Social Status.
In Assassin's Creed: Brotherhood, along with the previous symbols, new ones such as those for the Borgia towers have been added. Additionally, you are able to change the view of the map to show the division of districts, the percentage of completion in each district, and the completion percentage of Rome itself.
Apple of Eden Map
Altaïr views a map that is projected when he gets the Apple in AC I. He recreates what he saw within his Codex Pages. This can then be viewed in AC II. These maps have certain locations marked on them, these are arguably Pieces of Eden and/or locations of the Temples of Those Who Came Before. In Assassins Creed Brotherhood there is another map that shows some of these locations. It can be found on a wall in the Tiber Island Hideout (behind the table where you see recruits talking).

