what is principle or capacitor?

A small device used to store huge amount of electric charge in a small room is called capacitor. Take an insulated metal plate A. Charge the plate to its maximum potential. Now take another insulated plate B. Take the plate B nearer to plate A. You will observe that negative charge will be produce on the plate near to plate A and the same amount of positive charge will be produced on the other side of plate B. Now the plate B will start affecting the plate A slowly. The negative charge will start decreasing the electric potential of plate A. But positive charge helps in increasing the potential. But the effect of negative charge is much more than that of the positive because the negative side of plate is near to the plate A. So potential of A will start decreasing and it can be charged again to raise its potential to maximum.