When you completed a mission, you get the victory screen with below that a Score. In single player, this score doesn't really matter, it's just something for yourself: if you replay that mission, you can aim for a higher score than the previous go through.
In multiplayer, however, your score determines your rank on the player list of that game.
How is that score calculated?
Your score largely depends on the mission goals: if something is a goal you will get points for it, if not, you won't get any points. Also, your score depends on how long it takes you to complete the mission: 2 points will be subtracted from your score for every month you spend in the mission.
Table with points:
|Goal type||Score if it's a mission goal||Score if it isn't a goal|
|Population||city population / 10||0|
|Treasury||city treasury / 25||city treasury / 50|
|Monument completion||percent complete x monument value (see below)||N/A|
|Menagerie||25 x different animal types in your palace||10 x different animal types|
|Trading cities||50 x trading partners||0|
|Vassal cities||250 x vassal cities||0|
|Allied cities||150 x allied cities||0|
|Housing level||100 per housing goal||0|
|Production||100 per production goal||0|
|Profit||highest annual profit / 25||0|
|Hero||10 for each month in which a hero is in your city||0|
Monuments are a little different: some monuments are very large, some are small. Thus you won't get the same amount of points for every monument completed. The following table lists the monuments and their points:
|Grand Temple Complex||300|
|Earthen or Stone Great Wall||500|
All points added up form your "Raw score", this score is multiplied by a value that depends on difficulty:
|Very Easy||Easy||Normal||Hard||Very Hard|