Why A capacitor is two conductive plates separated by an insulating dielectric. When voltage is applied, charge builds on the plates and the energy is stored in the electric field between them, with the amount given by E = 1/2 CV^2. Capacitance is measured in farads, and larger plate area or thinner dielectric means more capacitance.
Watch out The inductor is the tempting swap: it stores energy too, but in a magnetic field around its coil, not an electric field. A resistor stores nothing, it converts electrical energy to heat.
Capacitor = Charge = Electric field; Inductor = Iron core = Magnetic field.