Volts, Amps and Resistance is what you mostly have to work with on a car. If you know two of the three you can always figure the third with a simple formula. Volts over Amps times Resistance.
Volts divided by Amps = Resistance. Volts divided by Resistance = Amps. Amps times Resistance = volts
If in your travels you want to know the Wattage just figure Amps times volts = Watts.
First you need to understand what is happening when electricity is moving. First is "Which way is the current going in a circuit? (A circuit is a circle, battery to load back to battery)
There are two different theories on which way current actually travels. It goes down to the real basics. If an atom of a conductor has loose electrons in it's outer ring (- charged) then one or more of the loose electrons can be drawn over to a nearby atom that has a lack of loose electrons in it's outer ring (+ charged). Some say it is the loose electrons moving from (-) to (+) is the direction and others say that the lack of electrons (+) called the "Hole" is drawn over to the extra loose electrons and thus the travel is from (+) to (-).
However, it is of no use to you to worry which way it travels, it just don't matter in car electrics.
The battery stores the negative and positive charged atoms in it's plates. Waiting for you to connect a load (a light, an ignition system, an electric motor or anything electric) to the battery with a switch of some kind and the wires to conduct that current through the circuit. keep in mind that a complete circuit must be made for any current to move. The negative cable and body and engine block are used in this complete circuit on all cars. Any resistance in the complete circuit can and will cut down the amount of current that can travel all the way from the battery to the switch to the load and back to the battery.
The largest draw of current in all cars is the starter motor. Thus the battery cables must be large on both the + and the - cables and a large ground strap from the engine to the body. It also requires a very large switch to connect the battery to the starter motor. No, not the ignition switch. The start position of the ignition switch only operates a solenoid that pulls a large brass plate down to connect the battery cable to the starter motor itself. The solenoid itself draws a lot of current so a larger wire is used to trigger the solenoid. It takes a lot of current to make this battery connection but does not take much to hold it there, so there is a small button switch in the solenoid that changes the "Pull" coil in the solenoid to a "Holding" coil in the solenoid thus reducing the amount of current used to hold the starter in operation. If it were not for the starter motor you could use a very small wire on the (+) and (-) side of the battery.
The next heaviest draws (load) in a car are the head lights, charging system, brake lights, radiator fan (if electric) and the heater / AC fan, AC compressor clutch, windshield wipers, electric windows and the fuel pump. The lowest draw in a car is the dash lights, side marker lights, interior lights, radio, ignition system and instruments. All of this will be reflected in the size wires used to operate all this equipment and also the size of the fuses used to protect the system.