Alternator Diode

The alternator is a form of generator. When the engine runs, this generator provides electrically energy to charge the cars battery. This provides electrical energy for the electrical systems within the vehicle as well as keeping the battery fully charged. (a Dynamo generator is used occasionally but an alternator is fitted in most common vehicles.) The generator must change the mechanical (kinetic) energy into electrical energy.
In very simple terms an alternator works by electromagnetic induction (try researching faraday's laws) the mechanical energy in this system is used to rotate a magnet within a 'stator' (often soft iron) usually with a multi phase output, meaning more electrical energy is created more frequently.

The energy this alternator provides must be limited roughly to the voltage of the battery (around 14.2 Volts) however as the engine rotates faster, as will the magnet within the stator and so more energy will be produced which must be controlled to prevent damage to electrical components or overcharging.
An alternator diode is a solid device which allows current in the circuit to flow in only one direction and are highly sensitive to both heat and voltage therefore they are used to prevent surges which can damage the battery and regulator. This diode provides a similar protection to these kind of conditions as a conventional fuse protects against current surges within a common home plug.