Series Circuits
In series circuits, the current is the same at all points. Using an ammeter to measure the current, if you set up a circuit like the one above, all the ammeters would give the same reading.
Bulbs resist the flow of current. The more bulbs you add to a series circuit, the more resistance there will be. Less electric current can flow if more bulbs are added. Less electric current means that fewer electrons are passing a given point every second.
Resistance reduces the amount of current flowing everywhere in the circuit.
Parallel Circuits
In a parallel circuit, there are junctions in the circuit so the current can flow around the circuit in more than one way. In a series circuit the current decreases as more bulbs are added. In a parallel circuit, as more bulbs are added, the current increases. This is because bulbs added in parallel offer less resistance.
Think of a water tank with a pipe to drain the water from the bottom. If two pipes were added, twice as much water could drain out. In a parallel circuit, the current is not the same at all points. See the diagram above.
Electrical devices in our homes are connected in parallel so that we can control each thing separately from the other. Christmas lights are a classic example of a series arrangement. If one bulb goes, none of the lights will work. House lights arranged in parallel mean that each one can be turned on and off independently of each other.
You can take one bulb out of a parallel circuit and the other bulbs won’t go out because the electricity can still flow round the circuit.