The world is a big place, and you don’t always know what’s going on everywhere. Early civilizations certainly didn’t have any idea what the people on the other side of the mountains looked like until they sent out explorers; unless they put out sentries, they might not know that an enemy was massing a huge army just outside of their own borders.
In Civilization V, until you explore the world, it’s hidden in the “fog of war.”
The fog of war is represented by the white clouds that cover much of the world at the start of the game. As you move units around, the fog of war will pull back, revealing more of the world. Once you have uncovered the fog of war from a tile, it doesn’t come back. However, if a unit moves and you can no longer see a tile, you won’t know if anything is going on there.
The Three States of Knowledge
If a tile is currently visible to a unit or your territory, you can see its terrain, any improvements on it, if it’s within any borders, whether it’s part of a city, any unit which may occupy it, and so forth. Subject to technology limitations, you’ll see the resources in the tile as well.
If you have uncovered the fog of war from a tile but cannot see it at the present moment (because the exploring unit has moved away, for instance), the tile is slightly darkened. You can still see the terrain in the tile, but you will not see any units in the tile. You may not see any improvements, recently-constructed cities, and so forth. Basically, your information
about that tile may be well out of date.
Tiles under the clouds of the fog of war are totally unknown to you. You don’t know what kind of terrain they are, who occupies them, or anything else. For all you know they may be flowing with gold or patrolled by ninja dinosaurs. You’d better get someone out to explore them as quickly as possible!
What is Seen
You can always see everything within your borders, as well as one tile away from your borders. Most units can see everything within 2 tiles (except for tiles behind mountains and blocking tiles; see below). Units on hills can see over blocked tiles. Certain promotions will extend a unit’s sight by 1 tile, and a number of mid- to late-game naval units have extended sight as well.
Mountains and Natural Wonders are impenetrable: they totally bar visibility of what’s beyond for everything (except for flying units). Forests, mountains and hills are all “blocking” terrain. Units can see into such tiles, but they
cannot see past them – unless they occupy a hill. Units on hills can see over blocking terrain into the tiles beyond.
Some ranged units are capable of “indirect fire,” which
means that they can shoot at targets they can’t see, as
long as another friendly unit can see them. For example,
an Artillery unit can shoot over a hill at a target it can’t see
if a friendly unit is atop that hill.