A good example of a voltage source, and the most commonly used, is a battery. A voltage source is essential for any electrical circuit to function.
Batteries consist of two oppositely charged electrodes; a cathode at the positive end and an anode at the negative end, and an electrolyte solution. Voltage is generated when a chemical reaction between the electrolyte and electrodes causes electrons to move from the cathode to the anode. The electrons are then passed along an external electrical circuit and back to the cathode to continue the reaction. Placing a light bulb, for example, in the circuit causes the electrons to pass through the light bulb and light it. The higher the voltage, the brighter the light will burn.