From ibikeu Wiki

A power meter is a device which integrates to a bike's drive-train through either a special crank or hub and reads the amount of power being produced, in watts. Power meters are the most accurate way to determine one's on bike performance and are an invaluable resource for top tier racers. However, for the novice cyclist, using a power meter properly in training usually involves a steep learning curve. They are also very expensive pieces of equipment.