In parallel circuit,
The current splits up when it comes to a branch. The current in all the branches adds up to the current in the main part of the circuit.
If one bulb breaks, the bulbs in the other branch stay on.
If you add more bulbs, they stay bright. It is easier for the electricity to flow because there are more ways for the electrons to go.
In serial circuit,
The current is the same everywhere.
If one bulb breaks, all the others go off.
If you add more bulbs, they will be dimmer because it is harder for the electricity to get through - the resistance of the circuit is higher.