In a series circuit, the same electrical current flows between common nodes, whereas in a parallel circuit two common nodes share current, giving it more than one path through which to flow. Components in a series circuit have equal currents running through them while parallel components share the same voltage.
A node is an electrical junction between two or more components. In a circuit, current flows from a high voltage source to a low voltage target, taking every possible route towards the lowest possible voltage, usually ground. A series circuit allows the current to travel between nodes without changing its path or being split towards other directions. A parallel circuit occurs when two components share two common nodes, creating a situation where some current flows in one direction and the rest flows in another.
Since parallel circuits can support arbitrary numbers of components, each one reducing the overall voltage of the circuit, electricians increase their use of resistors to compensate for Ohm's Law. Wiring more than two resistors on a simple parallel circuit results in equivalent resistances based on the divided sum of the components' resistances. For instance, in a parallel circuit, it is possible to wire four 10 Ohm resistors in such a way as to produce 2.5 Ohms of resistance in total.