The capacitor is an electronic device which has the capacity to store charge. Basically, a capacitor is nothing more than two conducting surfaces at a distance from each other with a dielectric material (or insulating material) in between. The charge is stored in the form of electron deficiency on one surface as compared to the excess of electrons on the other, thus this imbalanced electron creates a potential difference across the surfaces.
The characteristic of electrical energy storage in a capacitor is known as capacitance and is measured in terms of Farads (named after the British chemist and physicist, Michael Faraday). One farad of capacitance is defined as that amount of capacitance which causes one coulomb of charge to create a potential difference of one volt across the conducting surfaces of the capacitor.
For a given capacitor, the ratio of charge on the plates to the potential difference developed across them always remains constant. Whenever a capacitor is connected across a source of emf, its conducting surfaces develop a positive charge on one plate and a negative charge on the other and this in turn creates an emf in proportion to that charge. There is no flow of current through a capacitor for the reason that it does not have a conductor connecting both plates; there is an insulator or a dielectric between them.
If you charge a capacitor and leave it like that, theoretically, it will never discharge since there is no way for charge to move between the plates. But practically speaking this is not so and there is always a small leakage current due to imperfect dielectric material properties. Still, a good quality capacitor can hold the charge for several weeks if not months before losing its charge.
There are several types of capacitors depending on the material from which they are manufactured and we will study this in a different article. Capacitors can also be connected in series and parallel just like resistors but their combination follows different rules for total capacitance which will also be studied separately.