The Fog of War is a feature in the Command and Conquer series that prevents you from seeing certain units and structures. All areas of the map that's not immediately within any of your units' sight ranges and not covered by the shroud is the Fog of War. It is to represent the uncertainty produced by gaps in military intelligence.
Unlike the shroud the player can see the terrain in the fog of war clearly, while structures shown as they were last seen (whether of not they've changed since then cannot be determined until another of your units gets with sight range of it again) and units are not shown at all.
Commanders must be careful on leading forces into the Fog of War because there might be enemies in the fog. Commanders can also take advantage of the fog of war (though not against AI opponents) by putting long-range units along with spotters and some tanks to ambush enemies that may be passing by.
It is recommended that a commander send scouts to prevent enemies maneuvering unnoticed in the fog. Aircraft are also recommended in games from Generals onward because of their huge sight. However, in earlier games, planes do not have a very large sight radius, so they are not recommended.
In earlier games, fog of war is only an option in skirmish and multiplayer modes. In campaign modes, when shroud covers an area is dispersed, this area will be clearly seen permanently. However since Generals, due to increased number and effectiveness of support powers (especially offensive powers), also the tremendously enhanced superweapons, fog of war is added to campaign modes and is always activated in both skirmish and multiplayer modes, for consideration of balancing. Commanders not only unable to keep an area clearly seen without placing units or structures within its vicinity, also cannot aim support powers on a position if it's covered by fog of war.
Still, superweapons are not obscured by fog of war, which means a commander can call down superweapon attack if he/she acknowledges targets' whereabouts inside fogs. And superweapons have the side effect of temporarily removing fog of war, so offensive powers can be called down as follow-up attacks.
Some trainers are used to remove fog of war in later games, for its absence is a major characteristic distinguishing Command & Conquer from other RTS franchises such as StarCraft.