Example of differing sight areas.
The Shroud of Darkness, or SoD, is a gameplay mechanic which prevents you from seeing into places you do not have units present. Unlike Fog of War, which prevents you from seeing into places you haven't been yet, Shroud of Darkness lets you see the lay of the land, such as resources and paths, but not any enemy units that there may be.
Engines[]
In the original Glest game, you can turn the Shroud of Darkness (and Fog of War with it) off in the INI. In GAE and MegaGlest, this can be done on the new game screen.