A capacitor is an electronic device which stores an electric charge. When used with AC power, they introduce a phase-delay on the voltage. They come in all sizes and shapes and materials, but all are rated based on two physical properties: capacitance and voltage. Capacitance is measured in Farads, or more commonly, micro-farads and is a measure of the amount of electrical charge it can hold. Voltage refers to the highest voltage the capacitor can withstand before breaking down, as measured in volts.