Commerce

§ Commerce is the lifeblood of a civilization. It allows you to pay for your cities, and for the people and buildings inside them. Commerce lets you maintain standing armies. It lets you purchase trade goods from other civilizations, and it can also let you bribe those civilizations to keep them from grinding you into a pulp. Money matters.
§ See Wealth for an in-depth look at money in Civilization IV.
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Commerce Summary

§ Generating Commerce: Your civilization generates commerce when your citizens work the lands around your Cities. Tiles that bring in commerce are represented by a coin icon on the city screen.
§ Terrain: Some Terrain-Types generate more commerce than others. Water tiles and the land next to rivers tend to generate the most commerce.
§ Improvements: Your Workers can Improve land to make it generate additional commerce. The Cottage improvements are one of the best ways to bring in more commerce.
§ Resources: Some Resources also provide commerce, once improved and connected to your cities. Gems, gold, and incense in particular are worth a great deal of commerce.
§ Buildings and Wonders: Constructing certain Buildings and Wonders can increase commerce in a city.
§ Trade: You can Negotiate trade deals (particularly Open Borders) to increase commerce.
§ Expending Commerce: You spend your money in a variety of different ways. See Wealth for more details.