Defined
An electric generator is a device that converts mechanical energy to electrical energy. So, if we think of a wind turbine, the mechanical energy of the blades turning around, spinning a central axle will cause a mechanical motion that can be transferred into electrical energy via the electric generator.
source: http://ga.water.usgs.gov/edu/hyhowworks.html
So, if we expand this knowledge, we can identify many other ways mechanical energy can be converted into electrical energy with:
- turbine with steam, water, or wind
- hand crank
- combustion, reciprocating or piston engine
- magnetism
The rapidly growing field of electromagnetism has spawned many electromagnetic generators, some with technology that has been around since electricity was identified!