Background Info Electrical circuits are all around us and contribute greatly to the technology used today. Most gizmos, gadgets and devices will require some type of battery for operation. D.C. or direct current circuits are commonly found for applications that have the main power source located near by. Batteries are the most common voltage source for DC circuits because they can store a large amount of energy while not taking up much space. The term D.C. implies that the current generated in the circuit does not fluctuate inversely, meaning positive to negative. In an A.C. (alternating current) circuits, the current fluctuates inversely and at a very rapid rate. This is due to the voltage alternating positive to negative repeatably. This is best seen in a sinusoidal wave form.  http://blog.gogreensolar.com/2015/02/ac-vs-dc-breakers.html One very common example of a multi-system that uses numerous D.C. circuits is a vehicle. Vehicles have one main source, the battery and one charging source, the alternator which is actually a rectified A.C. generator. Multiple loads are spread throughout the vehicle such as: the headlights, the starter, the fuel pump, the turns signals and various others. All these loads can be represented as a resistor since all have some value of resistance associated with them. https://www.howacarworks.com/illustration/92/a-typical-car-electrical-system.png In a basic D.C. circuit, three things are needed for it to work. Those elements are Voltage, Current and Resistance. Without any of the three, the circuit will not function. The following circuit shows a basic interpretation of how a vehicle's headlights are wired. The two lamps on the right side each have a certain amount of internal resistance (3 ohms) which glow and produce a light when current flows through them. Note that the source is a 12V (volts) battery and both lamps separately have a resistance of 3 ohms. A current of  4A (amps) can be seen passing through each lamp. The source has a current of 8A passing through it. The relation between voltage, amperage and ohms are tied together under the equation of Ohm's Law: E = I x R. Ohm's Law was discovered by Georg Ohm, a German physicist. http://media.fluke.com/images/6004178-dmm-whatis-ohm-2-715x360.jpg The question may be asked how the total current was calculated. Resistors behave differently when connected end to end (series) or when all the resistors connect strictly to two points (parallel). The following picture shows how to sum the resistor values from the two scenarios. https://i.stack.imgur.com/I3HNw.gif Using the parallel equation, the resistance equivalent can be calculated. 1/ [1/3 + 1/3] = 1.5 Ohms  With the new resistance value the current leaving the battery can be calculated. 12 Volts / 1.5 Ohms = 8 Amps This is the first step in circuit analysis.