A capacitor in essence contains two conducting surfaces separated by a layer of an insulating medium called dielectric. The conducting surface can be in the form of either circular plates or be of spherical or cylindrical shape. The reason of capacitor is to store electrical energy by electrostatic stress in the dielectric.
It is described as, " the amount of charge needed to create a unit potential difference among its plates".
If we give Q coulomb of charge to one of the two plates of capacitor and if a p.d. of V volts is established among the two, then its capacitance is,
C = Q / V = Charge / Potential Difference