An electric circuit is simply a connection of electrical elements. A light bulb connected to a battery is a basic circuit. The battery delivers voltage to the system, and the light bulb introduces resistance. If the circuit is closed, meaning that it creates a complete and unbroken loop between the two terminals of the battery, the voltage will cause current to flow through the circuit and the light bulb will be illuminated. Energy is therefore transformed by the circuit from electrical potential energy in the battery into electrical kinetic energy flowing through the conductors, which is then transformed into light and heat in the lightbulb. In essence, the goal of every circuit is to transform electricity into some other useful form.
All the power that is provided to the system must also be dissipated by the system because of the law of conservation. In this case, the battery is the "source" and the light bulb is the "load," meaning that the battery supplies the power and the light bulb uses it. When investigating the electrical impact of a load on a direct current (DC) circuit, it is often sufficient to consider the load's resistance, and so for the sake of simplicity, loads are often characterized only by the resistance they introduce. We can then create more convenient circuit models, dealing simply with voltage sources and resistors.
In the case of a simple DC circuit, a DC voltage is applied across the circuit, which causes current to flow by conduction. This occurs because the terminals of a voltage source, such a battery or photovoltaic module, have different electrical potentials, causing the flow of charged particles from the point of higher electrical potential energy to the point of lower electrical potential energy. Or put another way, the negatively charged particles at the negative terminal of the voltage source are attracted to the positively charged particles at the positively charged terminal, and so electrons begin to flow through the conductor connecting the two terminals. This electrical kinetic energy will do work on another system, and so if a device is connected between the two terminals, it can be operated by this flow of electricity.
In order for electrons to flow through a conductor, the circuit must form a complete and closed path. If the path is broken at any point in the loop, this is called an open circuit, and no current can exist, meaning that no power can be developed. Voltage can - and often does - still exist across an open circuit, much the way that there is a voltage across the two terminals of a charged battery. Therefore, there is the potential for work to be done, but no work is performed unless charge is allowed to flow.
The way that electrical elements are connected in an electric circuit dictates the way that those elements interact. For instance, charge is a conserved quantity. It is neither added to or removed from the closed circuit and only moves from one place to another. Much like water flowing through a hose, the electrons in the circuit all move at a uniform rate as electrons "push" one another down the wire. Therefore the current at one point in a circuit branch must be the same as the current at another point in the same branch. If all of the electrical elements are wired in one loop, the current at every location in that loop must be equal. We refer to this as series wiring.
In the event that a circuit has multiple branches, there is now more than one path down which current is able to flow. This point where multiple branches or loops of a circuit come together is known as a node, and by the law of the conservation of current, the current entering any given node must be equal to the current leaving that node. Charge will tend to travel down the path of least resistance, and so the way that current splits at a node is dictated by the amount of electrical resistance in each branch of the circuit. We refer to this method of wiring together electrical elements as parallel wiring - where different elements are in separate and parallel branches.