An alternator is a device that generates AC power from the rotation of an axle (mechanical energy). They are a bit similar to dynamos, but differ in the fact that they make AC power, whereas dynamo's generate DC power. In addition, they also tend to be more efficient.
Several types of alternators exist, some requiring permanent magnets, others requiring only additional electrical energy (electromagnets). This is an important distinction as rare earth minerals are becoming ever scarser.
Depending on the type of alternator or dynamo chosen, the optimal efficiency can only be obtained in best conditions. These depend on the rotation speed of the axle. If this is too high or low, the conversion can not occur efficiently and huge power losses are the result.