Generators
The generators house a series of magnets. These magnets are rotating with the turbines past copper coils. The resulting magnetic field produces alternating current (AC) inside the generator. Below is a simple example of a simple generator using magnetic induction.
The strength of the magnetic field is stronger where the magnetic field lines are most dense. This means that the strength of the magnetic force varies depending on the position of the magnet. Induced emf (electromotive force, or voltage) is directly proportional to the magnetic field strength. This is the reason for the generation of AC voltage.
http://www.alternative-energy-tutorials.com/wind-energy/wind-turbine-generator.html
Transformers
Also inside the power house is a transformer, which amplifies the AC voltage produced by the generator. Transformers appear simple on the outside, but are more than meets the eye. They are essentially dual electromagnets. Transformers that increase current, like the ones used in hydro-power, do so by increasing the number of turns of wire on the output. Below is an image of a simple transformer:
http://www.school-for-champions.com/science/ac_transformers.htm#.VTRi8ZOgUT0
The figure demonstrates the relationship between the primary to secondary ratio of turns of wire and the same ratio of voltage.