A capacitor is an electronic device for storing energy in the form of charge. It acts a bit like a short-term battery by storing charge when an electro-motive force drives current into one end of it, and then supplying the stored current out of that same end when no such external electro-motive force exists.

In an analogy based on hydraulics, a capacitor is somewhat like a barrel with a rubber membrane stretched across the middle of it. Water can be forced into one end of the barrel, thereby stretching the membrane toward the other end. The water pressure is essentially stored up by the deflection of the membrane. When the pumping force is removed, the rubber membrane pushes water back out of the same end through which it previously entered. In this analogy, water is like electric charge, the movement of water is like electric current, and water pressure is like electric voltage.