A capacitor is defined as two conductors seperated by an insulator. This could be as simple as two sheets of tin foil held apart and connected to a power source.

The capacitance of a specific capacitor is defined as:

Where k is the dielectric constant of the insulator, eo is the permisstivity of free space, A is the overlapping area of the plates, and d is the distance between the plates. Capacitors store electric charge.