What is a capacitor

A capacitor is a device for storing charge, made up of two parallel plates with a space between them. The plates have an equal and opposite charge on them, creating a potential difference between the plates. A capacitor can be made of conductors of any shape, but the parallel-plate capacitor is the most common kind. In circuit diagrams, a capacitor is represented by two equal parallel lines.

Definition Provided by Spark Notes

Introduction to capacitors

How do capacitors work

   Capacitance is a term used to describe how well a capacitor can store charge. The capacitance of a capacitor depends on several thing.  because a capacitor is a two plates separated by a distance then both the area of the plate and the distance between the two plate both effect the capacitance of a capacitor.  

    Capacitors stores electrical charge on two plates, if the plates were expanded in area, then it would seem the there is more room charge, therefor increasing the ability store charge. 

    To understand how distance between the two plates can effect the capacitance of a capacitor, we need to examine how a capacitor works.  

    The voltage pushes charge in a circuit until it reaches the capacitor. Because there is no connection from one plate to the other so the charge is stuck.  As more charges are pumped on to the plate, the electrons spread out evenly over the plate.   

  The charge creates an electric field which prevents more charge from enetering the plate (stoping the current eventually, it takes some time to stop the flow).  At the same time the electric field is pushing the charges off the second plate.  This continues to happen until the charge on one plate balances out with the charge on the second plate.

 At the same time the electric field is pushing the charges off the second plate.  This continues to happen until the charge on one plate balances out with the charge on the second plate.

So if the distance between plates is small then the electric field get up to strength sooner, with less charge, "reducing" the storage capacity of the capacitor (and reducing the capacitance).