Here's some more:
Resources
There are two types of resource, Commercial and Strategic Resources.
Commercial Resources
Every commercial resource in your territory provides gold. The amount of gold produced by the resource depends on the value of the resource, and how many instances of the same resource you have. Each successive resource of the same type produces less gold then the previous one.
For example, if a resource has a value of 6, the first instance of that resource provides 6 gold per turn, the second 3 gold, the third 2 gold and so on.
However, resources can be traded. This means that if you have more then one instance of the same resource (and so are receiving less then it's true value in gold, you can exchange this resource for something else with another player.
E.g. Player A has 2 saffron resources. Saffron has a value of 10, but overall they are only receiving 15 gold per turn. Player B has 2 ivory resources with a value of 10, but also receive only 15 gold per turn.
Player A and B decide to trade their resources so each player now has 1 saffron and 1 ivory. They both now receive 20 gold per turn. Both players gain 5 gold per turn from this arrangement.
Although this idea of diminishing returns reduces the value of successive resources of the same kind, controlling many of a resource can allow the player to build a monopoly on it's trade and so profit by exchanging these resources with other players.
Strategic Resources
Unlike commercial resources, strategic resources do not provide gold per turn, or have diminishing returns. They instead provide different effects. They can be traded in the same way as commercial resources.
Copper: Provides a discount to training infantry
Tin: Provides a discount to training infantry
Horses: Required to train chariots, and provides a discount to them
Hard Wood: Provides a discount to building ships.
Grain: This does not provide a concrete discount, but owning grain resources increases the rate of natural growth in your cities, as well as the effect of expansion projects, causing your population to expand faster.