A capacitor was formerly known as condenser. It is a device for storing electric charge. Practical capacitors are made using various methods. But in all of them there are at least two conductors separated by an insulation.

A pair of conductors separated by a dielectric or insulator forms a capacitor. If we apply a potential difference or voltage across these two conductors, then a static electric field is formed across the insulator or dielectric. Hence, positive charges are stored in a plate or conductor and negative charges in the other. The unit of capacitance is Farad. It is the ratio of electric charge to the potential difference on each conductor.