A wind turbine is a rotary device that extracts energy from the wind and converts into rotary mechanical energy. Blowing wind spins the blades on a wind turbine. When the wind blows, a pocket of low pressure air forms on the downwind side of the blade. The low pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift.
The force of the lift is actually much stronger than the wind’s force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller. The rotors of the turbine are attached to a hub that is mounted on a rotating shaft. The shaft goes through a gear transmission box where the rotating speed is increased.
The transmission is attached to a high speed shaft which turns a generator that produces electric power. The electric power may be dc or ac depending on the end user’s need. Direct current can be generated by driving brushless dc generator attached to the windmill’s rotor.
Simply stated, a wind turbine is the opposite of a fan. Instead of using electrical energy to produce wind, like a fan, wind turbines use wind to generate electrical energy.
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Wind turbines can be used as stand-alone applications, or they can be connected to a utility power grid or even combined with photovoltaic system. For utility-scale sources of wind energy, a large number of wind turbines are usually built closer together to form a wind plant. Stand-alone wind turbines are used for water pumping or communications.
The performance of a wind power plant is characterized by the annual energy generation related to 1 m2 of area swept by the rotor of wind turbine. Depending upon the average wind speed, the annual energy generation lies between 250 to 600 kWh per m2 rotor area. 
Wind turbines are designed to exploit the wind energy that exists at a location. Aerodynamic modeling is employed for determination of optimum tower height, control systems, number of blades and blade shape.
Conventional horizontal-axis wind turbines can be divided into three components:
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i. The rotor component includes the blades for converting wind energy to low speed rotational energy and costs about 20% of the total cost of wind turbine.
ii. The generator component includes the electrical generator, the control electronics and most likely a gearbox for converting the low speed incoming rotation to high speed rotation suitable for generation of electrical energy. It approximately costs about one-third of total cost of wind turbine.
iii. The structural support component includes the tower and rotor yaw mechanism and costs about 15% of the wind turbine cost.
Other equipment including controls, electrical cables, ground support equipment and interconnection equipment.
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Wind turbines are available in a variety of sizes and therefore power ratings. The largest machine has blades that span more than the length of a football field, stands 20 building storeys high and generates enough electricity to supply 1,400 homes. A small home-sized wind machine has rotor between 2.44 and 7.62 metres in diameter and stands upwards of 9.144 m and can meet the power requirements of an all- electric home or small business. Utility scale turbines range in size from 50 to 750 kW. Single small turbines, below 50 kW, are employed for homes, telecommunication dishes or water pumping.
Small wind turbines have speed in the range of 100-300 rpm while large wind turbines have speeds in 30-600 rpm range. Large wind turbines are employed for supplying generated power into the grid.