Series or parallel?

From physics you already know: in a series circuit there is only one path for the current, in a parallel circuit there are several. That is why the old string of lights goes completely dark when one bulb fails, while at home every lamp works on its own. In electronics, however, this picture is no longer enough: if you build circuits, you need to calculate exactly how voltage and current are shared.

In series: resistances add up

If two resistors sit one after the other, the current has to pass through both. The obstacles simply add up: 100 ohms and 200 ohms together make 300 ohms. The current is the same everywhere along the path, but the voltage is divided. Each resistor gets its share according to its size: the larger one grabs the larger part of the voltage, and all the partial voltages add back up to the source voltage. If all the parts in the chain are the same size, each one gets the same amount, so with three identical bulbs each gets a third. This is exactly how the series resistor in front of an LED works, by the way: it takes the part of the voltage the LED cannot handle.

In parallel: currents add up

Connected in parallel, the full voltage sits across every resistor, but the current splits between the paths. Each branch draws as much current as its resistance allows, and at the source all branch currents add up. The total resistance is the surprising part: it becomes smaller than the smallest individual resistor, because every extra path makes things easier for the current overall. Two equal resistors in parallel have only half the individual value together.

Why wall sockets hang in parallel

Now it becomes clear why your home is wired in parallel: every socket should deliver the full 230 V, no matter what is plugged into the others. If they were in series, the voltage would be divided between all devices and every plugged in device would change all the others. In parallel, each device works independently and draws exactly the current it needs. The currents of all devices add up in the supply line, and that is exactly where the fuse sits: if the sum gets too large, it switches off.

The toolbox for both cases

Remember the pattern as a pair: in series the current is the same everywhere, the voltages add up, and the total resistance is the sum. In parallel the voltage is the same everywhere, the currents add up, and the total resistance drops. With these rules and Ohm's law $U = R \cdot I$, rearranged for the current as $I = U / R$, you can work through any combination of the two circuits step by step.