In series circuits, the current is the same through each component. The potential difference of the power supply is shared between the components. A parallel circuit showing two bulbs, a switch ...

The circuit ... 2.2 K resistor in parallel with the LEDs. When turned on by the PIC, this transistor causes roughly a 10 mA current to flow through the Zener diode and its series resistor.

Two lamps connected in series with an open switch ... Two lamps connected in parallel with an open switch and a cell The circuit diagram shows a circuit with two lamps connected in parallel.