Capacitors.

What is a capacitor?
A capacitor is a device which can store electrical energy in an electrostatic field. This energy can remain stored for some time after the supply has been removed from the device. For this reason, some high value capacitors can be dangerous when used with high voltages.
One such capacitor, which stores a high voltage (25,000 volts) for some time after power is removed, is found in television sets. This capacitor is formed on the rear of the screen, by the glass and coating used. Great care must be taken in working with such circuits.

Values
As with resistors, capacitors come in a wide range of values. The unit of capacitance, the Farad, is a rather large value - so smaller units are used.
The units used range from microFarads (10^-6 Farads) down to picoFarads (10^-12 Farad).
Capacitor values can range from just one or two picoFarad up to 1000s of microFarads.

Types.
There are different types of capacitors, made from different materials.
Some of these different types, such as electrolytic, make higher capacitance values possible. Other types may be cheaper, have a lower leakage, or be better in a particular use.

Dissipation.
In normal use, capacitors should not dissipate power as heat. The values to look for are capacitance and voltage - capacitors should be rated at a higher voltage than would be expected in the circuit, even considering some possible fault conditions where possible.

Schematic symbol.
In circuit diagrams, capacitors may be shown using one of the symbols on the right. The variable capacitors, shown with an arrow, are usually user controls and available to the user. The preset types are usually inside the equipment, designed to be adjusted once and then left alone.