Fog of war

Fog of war is the idea that in war you have little knowledge of enemy actions that occur outside your line of sight.

In Empire Earth this is portrayed in that any place on the map that no friendly units have visited is obscured (black), so you can't see any units, buildings, or terrain in that area. When a friendly unit visits a location, the area within its line of sight comes in to full view (brightly lit), and you are able to see everything in the area. When the friendly unit(s) are no longer in that area, the area becomes darker (shaded) and you can only see the terrain and buildings that were there at the time the unit(s) were there.

Depending on the setting, the reveal map option for hosts is used so the total blackness is gone, making the terrain and each player's starting capitol visible to all players.

When diplomacy is involved, players who are allied with each other share explored areas of the map and line of sight.

The Library of Alexandria wonder will make all enemy buildings fully visible (brightly lit), as if there is line of sight on the building location.

The Pharos Lighthouse wonder will grant you line of sight, only in water, within a certain distance of the wonder.