Capacitor & Capacitance

The capacitor is a device that can store an electrical charge (Q). It is composed of two conductive plates separated by an insulator (also called a dielectric). Connecting leads are attached to the parallel plates. The amount of charge that a capacitor can store per unit of voltage across its plates is called capacitance, (designated C). The greater the charge that a capacitor can store for a given voltage, the higher its capacitance value.

 

The Farad (F) is the basic unit of capacitance. Most capacitors that are used in electronics have capacitance values in microfarads (µF) and picofarads (pF). 

Charge Storage

In a neutral state, both plates of a capacitor have an equal number of free electrons. When a capacitor is connected to a DC voltage source through a resistor, the source moves electrons away from plate A through the circuit to plate B. As plate A loses electrons and plate B gains electrons, plate A becomes positive with respect to plate B. Remember that no electrons can flow through the insulator. The movements of electrons stop when the voltage across the capacitor equals the source voltage. The charge remains in the capacitor if it is disconnected from the source

Energy Storage

The opposite charges on the plates of a capacitor create many lines of force. They form an electrical field between the plates that store energy within the dielectric. The greater the forces between the charges on the plates of the capacitor, the greater the amount of energy stored.
The amount of stored charge is directly proportional to the voltage and the capacitance (Q = CU). The amount of stored energy depends on the square of the voltage across the plates of the capacitor.

You can more education at  http://en.wikipedia.org/wiki/Capacitance