One of the first principles to understand when you are learning about electricity is the distinction between a parallel circuit and a series circuit. Both types of circuits power multiple devices by the use of an electrical current flowing through wires, but that's where the likeness ends.
To understand the distinctions between a circuit where the devices are wired in series from one where they are wired in parallel, you must first understand the basics of an electrical circuit.
In very simple terms, all circuits work by providing a closed-loop of wires through which electrical current can flow. Electrical current is essentially the movement of electrons through the circuit from the source (through hot wires) and back to the source (through neutral wires). When lights or other devices are tapped into this circuit loop, the moving current can power those devices. Any interruption in the pathway (such as a switch being opened) stops the flow of electrical current—momentarily ending the circuit.
Circuits Wired in Series
In a series circuit, the devices along the circuit loop are connected in a continuous row, so that if one device fails or is disconnected, the entire circuit is interrupted. Thus, all devices along the circuit stop working at the same time. Series circuits are somewhat rare in house wiring, but they are sometimes used in strings of Christmas lights or landscape luminaries, where one light bulb failing will cause the entire string to go dark.
Circuits Wired in Parallel
Much more common than series circuits are those wired in parallel—including most household branch circuits powering light fixtures, outlets, and appliances. Here, the wiring is configured so that each device is in constant contact with the main circuit pathway. Individual devices merely "tap into" the main circuit loop, much the way free-way ramps allows cars to exist and enter a freeway without interrupting the main highway. A parallel circuit has many such "off-ramp/ on-ramp" loops, so that a failure in any single loop never shuts down the entire circuit.
Examples of Parallel Circuits
Most standard 120-volt household circuits in your home are (or should be) parallel circuits. Outlets, switches, and light fixtures are wired in such a way that the hot and neutral wires maintain a continuous circuit pathway independent from the individual devices that draw their power from the circuit.
Sometimes this continuous pathway is creating by "pigtailing" into the circuit wires in order to power an outlet or light fixture (the pigtails are the exit and entrance ramps for the current flow). Other times, the design of the device creates the continuous ongoing pathway. A standard outlet receptacle, for example, has a metal strip (connecting tab) between the pairs of screw terminals that assures that the pathway to the next outlet is maintained. If the receptacle goes bad, the connecting tab on the device assures that the current flow continues to the next outlet in the circuit.
A Practical Use for Series Wiring
One household example where series wiring is useful in when a single GFCI (ground-fault circuit interrupter) receptacle is used to protect other standard receptacles located "downstream" of the GFCI.
A GFCI receptacle has screw terminals labeled "line," as well as screw terminals labeled "load." The load terminals can be used to extend wiring to additional regular receptacles beyond the GFCI, which allows them to also enjoy GFCI protection. However, should the GFCI go bad, then all the connected downstream outlets will also cease to function. This section of the circuit, then, is an example of wiring in series.