The Harbor is a city improvement in the computer game Civilization II. It was not present in the first Civilization, and its addition to the second game has a surprisingly major influence on the game play.
One of the major problems with the first Civ, was, for all of the improvement you can do to land squares, ocean squares couldn't be improved on. So a city that was built on a peninsula or a small island could never get enough food or resources to grow very large.
The Harbor, which can be built fairly early in the game, and fairly cheaply, changes that. It allows each ocean square to generate one additional food, which means that most squares will be generating two food instead of one.
Apart from being a good source of food, this actually has a ripple effect for that city. Since oceans generate more trade then land, and he harbor gives the ability to utilize these ocean squares without losing food production, the net influence of the harbor is a great increase in trade. And with trade, the city can gain more science, tax and luxury. In the end, the harbor can be one of the biggest differences between happy growing cities; and cities in unrest with famine.
Other then the temple, which is a neccesity (especially at higher levels of game play), the harbor is one of the most important city improvements to build, up until your civilization switches to a more advanced form of government, and its cities start needing a large number of improvements.