A component that has an appreciable capacitance. It consists of an arrangement of at least two conductors or semiconductors separated by a dielectric (an insulator). The conductors or semiconductors are known as electrodes or plates. The value of the capacitance of a given device depends on the size and shape of the electrodes, the separation between them, and the relative permittivity of the dielectric. Most types of capacitor have a value, which may be variable, determined by the geometry of the device. Some capacitors however have a value that is also a function of the voltage across the device or of the operating frequency. The dielectric material may be solid, liquid, or gaseous. In circuit analysis, capacitative effects are often treated via a lumped approach, where the properties of an ideal capacitor are described by the time-domain relationship
In the Laplace domain this becomes
where is the Laplace impedance of the capacitor, and any charge stored on the capacitor at start-up is included naturally in terms of an initial voltage . See also ceramic capacitor; chip capacitor; electrolytic capacitor; metallized film capacitors; metallized paper capacitors; mica capacitor; MOS capacitor; plastic-film capacitor; varactor.
http://www.iequalscdvdt.com/index.html An introduction to capacitors
http://electronics.howstuffworks.com/capacitor.htm/printable Capacitors explained, on the howstuffworks website