Exploration

§ At the beginning of a game, most of the world is unknown to you. It is important to explore the world, to discover resources and sites to build new Cities and to encounter other civilizations. You explore your world by sending units out into the unknown: the spaces become visible once your units can "see" them.
§ There are three different kinds of spaces in Civilization IV: unexplored spaces, visible spaces, and explored spaces.
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Unexplored Spaces

§ Unexplored spaces are black. You don't know what terrain they contain, whether they are occupied by units, or anything else.
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Visible Spaces

§ Visible Spaces are very bright. They are all the spaces that your units and cities can currently "see." Spaces within your cultural borders are always visible. Your units and cities can always see the squares they occupy and those directly adjacent to them; if on hills, if looking across water, or if you have certain technological advances, they can often see farther. When you look at the map, you can see everything that any of your units or cities can see: other units, visible resources, improvements, cultural borders, and so forth.
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Explored Spaces

§ Explored spaces are darker than visible spaces, but not as black as unexplored spaces. These are spaces that you have seen in the past, but which none of your units or cities can see at present. Explored spaces are displayed on the map in the state you last saw them: you do NOT see any changes (new cities, improvements, roads, so forth) which have taken place since you last actually saw that space. You cannot see units in explored spaces.
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Map Trading

§ When you trade Maps with another civilization (see Diplomacy), all of the newly-visible territory is "explored" to you - you see the territory as your trading partner last saw it, but you don't see any units occupying those spaces. If your trading partner then builds new cities or improvements in these spaces after trading you his or her map, you will not see the changed conditions unless you trade for their map again.