what is the wind power theory?

Differences in atmospheric pressure due to differences in temperature are the main cause of wind. Because warm air rises, when air fronts of different temperatures come in contact, the warmer air rises over the colder air, causing the wind to blow.
Wind generators take advantage of the power of wind. Long blades, or "rotors", catch the wind and spin. Like in hydroelectric systems, the spinning movement is transformed into electrical energy by a generator.
The placement, or "siting" of wind systems is extremely important. In order for a wind-powered system to be effective, a relatively consistent wind-flow is required. Obstructions such as trees or hills can interfere with the rotors. Because of this, the rotors are usually placed atop towers to take advantage of the stronger winds available higher up. Furthermore, wind speed varies with temperature, season, and time of day. All these factors must be considered when choosing a site for a wind-powered generator.
Another important part of wind systems is the battery. Since wind does not always blow consistently, it is important that there be a backup system to provide energy. When the wind is especially strong, the generator can store extra energy in a battery.
There are certain minimal speeds at which the wind needs to blow. For small turbines it is 8 miles an hour. Large plants require speeds of 13 miles an hour.