A device used to store electric charge is known as capacitor.
A capacitor consists of two thin metal plates, parallel to each other separated by a very small distance. The medium between the two plates is air or a sheet of some insulator. This medium is known as dielectric.
If +Q amount of charge is given to plate A of a parallel plate capacitor, due to electrostatic induction, it would induce -Q charge on the inner surface of the plate B and +Q on its outer surface. The lines offer between the plates have been shown in the figure. ,
There exists a force of attraction between the charge +Q stored on the plate A and the charge -Q induced on the inner surface of plate B. Due to this force of attraction, the charges are bound within the plates and remained stored for a long period.