What is an alternator and how does it work?
An alternator is a device that converts mechanical energy into electrical energy . In automotive applications an internal combustion motor is used as your initial energy source. In power generation for your home other energy sources can be used such as impounding water in hydro electric dam power generation. Water is also used to make steam to power the alternator such as in nuclear power or natural gas fired operations as well. Many big cities have "Waste to Energy " which are large incinerators that burn trash to make steam to turn large turbines which in the end turn an alternator. 
No matter how they power the alternator they are usually all the same. Alternator are made up of two magnetic fields , the outer fields which is called the stator , you can usually see the stator winding through the cooling holes in your alternator. And you have your inner field which is called the rotor . When the rotor field spins inside of the stator field this reaction of the two fields breaking what they call the flux lines of magnetism causes power to be induced into the stator in the form of AC current . Alternators for cars or for large power generation plants are all three phase AC with the exception that some marine, motorcycle , atv applications use only single phase although some use three phase. All alternators make AC (Alternating Current). In power generation for homes the power stays AC and is sent out to homes and factories In vehicle applications the AC current is converted to DC (Direct Current) by the use of diodes in what is called the BRIDGE RECTIFIER . The Bridge Rectifier has Diodes which are one way gates that allow power to flow in only one direction. Thereby converting AC current to DC current. In power generation for homes they control the current by turning the alternator at constant speed . In automotive applications where the motor runs at different speeds they use a VOLTAGE REGULATOR which varies the power to the center rotor. If the voltage regulator sees that the alternator is turning slow or the vehicle needs more power it sends more power to the rotor making it a large electromagnet which makes the alternator produce more power. If your battery fills up or the vehicle doesn't need as much power the voltage regulator reduces the power sent to the rotor making it as small electromagnet making the alternator produce less power. 

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