Currents.
In a series circuit the same current flows through each part of the circuit. There is only one path for the current to flow round. Circuits A and Bare series circuits.
In a parallel circuit the current divides when ir comes to a junction. Part of the currents flows through the other branch. Part of it flows through the other. The bulbs in circuit C and D are joined in parallel.
When two branches have the same resistance, the same current flows through each branch