What is an alternater?

Early automobiles had a "generator" to keep the battery charged. In the generator a magnetic field was established and the armature spun inside the field, creating the electrical current. Carbon brushes made contact with the armature and the electrical energy was transferred to the battery and other electrical devices. The generator produced "Direct Current" and was controlled by a set of relays called a "voltage regulator".

Shortly after the wide acceptance of the transistor and other solid state devices Detroit came up with the idea of using an alternator. In the alternator, the armature becomes the magnetic field and spins inside a set of coils. Alternating current is created as the magnets pass the coils, then the alternating current is converted to direct current through an elaborate arrangement of diodes on a "diode plate"

An alternator has many advantages, it's much more efficient, takes up less space, is capable of producing 110 volts (if you know how to wire it up you can plug things in) and it produces significant electrical current even at lower engine speeds.

The alternator too is controlled by a voltage regulator to keep it from producing excessive voltage and current, but modern voltage regulators are solid state and typically inside the alternator.