Wikipedia: Capacitor

A capacitor is a device that stores electrical charges. A capacitor consists of two conductive plates at different electrical potentials, and an insulating dielectric or insulator. The capacitor's capacitance (C) is a measure of how much voltage (V) appears across the plates for a charge (Q) stored in it:

 C = Q / V

A capacitor has a capacitance value of one farad when one coulomb of charge causes a potential difference of one volt across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarads or picofarads.

The size of the charge that a particular capacitor can hold is:

 C = E_o * K * A / D

where C is the capacitance in farads, E_o is the electrostatic permittivity of vacuum or free space, K is the dielectric constant of the insulator used, A is the area of the each of the two plates, and D is the distance between the plates.

For electronic devices such as radios that select a particular frequency, the frequency selected is a function of the inductance (L) and the capacitance (C) in series, and is given by

 f = 1/(2 * pi * sqrt(L * C))

This is the frequency at which resonance? occurs in a series RLC circuit.

In semiconductor [intergrated circuit]? devices these capacitors are made up of metal lines and insulators on a substrate. They will store charges such as a HIGH or a LOW. For this reason, dynamic RAM (DRAM) was developed which stores bits in periodically-refreshed capacitors.

The amount of "resistance" of a capacitor to AC is known as capacitive reactance, and varies depending on the AC frequency. Capacitive reactance is given by this formula:

Xc = 1 / (2 * pi * f * C)

where:

Xc = capacitive reactance, measured in ohms
f = frequency of AC in hertz
C = capacitance in farads

It is called reactance because it reacts to change in the value of the current.

Since the reactance is inversely proportional to the frequency, capacitors completely block direct current, and for high frequency alternating currents the reactance is small enough to be considered as zero in approximate analyses.

Capacitive reactance is the negative imaginary component of impedence?.