When a loop of wire turns inside a magnetic field, a voltage or EMF is generated, according to Faraday's Law. If a circuit is present, a current flows. The induced EMF is proportional to
The rate at which the coil turns.
The number of turns on the coil.
The strength of the magnets.
This is the basic principle behind electricity generation. In a power station the coil is attached to a turbine. The turbine is turned by allowed steam to blow the a windmill like mechanism.
A typical power station may produce 1000MW of power in this way.
AC is the most conveniently generated form of electricity. It also allows voltages to be transformed up for long distance transmission,resulting in large energy saving, then transformed down to a safer voltage convenient for use in the home. The electricity from any socket in your house is AC.