In this posting, there are three types of circuits; series, parallel, and complex, using series and parallel.
A parallel circuit occurs when the path of the current is split. It provides a diffent path for the electrons to take in case, as an example, on bulb is removed...
... then the other path is open and the electrons go through that path. This means that the total current in a parallel circuit is equal to the sum of the currents in the resistors. But in a parallel, unlike the series, the voltage remains the same for each resistor; the total voltage is equal to the voltage of resistor 1 and equal to the voltage in resistor two. The voltage is the same as it is at the source. It is if it's a single wire with voltage running through it. Since voltage is the potential difference between two resistors, for all intents, there is no second resistor. The risistance in a parallel circuit is equivilent to the average of the two resistors. If you add more resistors, then the overall resistance is less and the current is greater, but with only two resistors, you get a smaller current. The equivalant resitance is equal to the amount needed for one resistor to 'do the job'.
A complex circuit means a blend of series and parallel. In plane words, this image shows two bulbs in parallel, connected to another bulb in series. You may notice that in this image the bulb in series is brighter than the two bulbs in parallel. That is because the current in the total parallel is equal to the current for the bulb in series. Once the current is in parallel, it changes depending on the resistance, like any other parallel circuit. To find the current and voltage for the resistors, you do the equations for for parallel first, and then imputing the results into the equations for series.
If the bulb on the right is removed, then the whole circuit goes out because it is series. If the bulb on the top is removed, then the two remaining bulbs become a series becuase the once-parallel circuit provided an alternate path.
In the picture below, the voltage will be explained.
The voltage in this complex circuit occurs when the total voltage in the series section equal to the voltage in the other path. The total voltage is equal to the voltage at the source. The resistance in the series section is double to the resistance on the other path, asuming the resistance for the light bulbs are the same.