Basic Electrical Charging Systems
Newer motorcycles use an alternator which generates AC electrical current. Older bikes had generators which made DC electrical current. Generators requred brushes which needed frequent replacement. Alternators have a rotor "magnet", which attaches to your crankshaft, and a stator which is a coil around the rotor. The spinning rotor generates an electrical field in the form of AC voltage.
The faster the alternator spins, the more electricity it generates. Therefore it is neccessary to limit or "REGULATE" the amount of voltage an alternator can generate. This device is called a "voltage regulator". They generally limit the maximum voltage of an alternator to around 16 to 17 volts.
The voltage generated by the alternator is still in AC and needs to be converted to DC, to be compatible with your bike's electrical system. A rectifier converts AC voltage, to DC voltage, by using diodes to control the direction of electrical current flow. Think of diodes as check valves for electricity if you will. They allow electricity to flow one way through them, but not the other. This allows you to convert "Alternating current", to "Direct current".
This simple system sends a steady supply of DC voltage to your electrical system and keeps your battery charged.