Units are used to measure the characteristics of an electrical circuit. The following three characteristics, Voltage, Resistance, and Current, are used frequently when working on automotive circuits. Of course, there are numerous other charactertics that can be measured, but we will just focus on the characteristics that will be leveraged most of the time while dealing with automotive circuits.
Voltage, measured in volts, Potential Difference
I like to think of voltage as potential difference. What better to explain potential difference than a roller coaster drop. It is clear to see that the taller hill on the right will cause the roller coaster car to travel further and therefore have more time to accelerate. The roller coaster on the right has more potential than the roller coaster on the left.
Voltage is the potential difference between any two points in a circuit.
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Current/Amps - Flow of electric charge
I like to think of amps as amount of electricy moved/used. To stay with the same simile, look as current as the amount of cars making up the rollercoaster. The more cars used the more riders are moving through the ride.
Amperage, measured in amps, is a measure of the AMOUNT of electricity used.
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Resistance - Opposition to Current Flow
I like to think of resistance as a kink in a garden water hose. So, the more a person kinks a hose, the less water comes out the end of the hose due to greater resistance in the hose due to the kink.
Resistance, measured in Ohms, is the measure of opposition to electric current.
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GeekDIYMechanic - Basic Automotive Electronics and Circuit Overview