When you build Coal powerplants, Hydro powerplants, Nuclear powerplants etc. They provide the city with 'power'. But what does the power actually do? Is it additional hammers or something? So when you build the Three Gorges Dam, it provides all your cities on the continent with power, which means what?

Power is just another hammer multiplier like factories or forges. +50%, I believe.

Heres a little article about power plants.

http://www.civfanatics.com/civ4/strategy/power_plant.php

Thank you very much. :)