A capacitor is an electronic component
that takes advantage of the ability of
electric fields to reach out across an
insulator.

It consists of two flat plates
made from a conducting material such
as silver or aluminum, separated by a
thin insulating material such as Mylar or
ceramic. The two conducting plates are
connected to terminals so that a voltage
can be applied across the plates.
Note that because the two plates are
separated by an insulator, a closed
circuit isn't formed. Nevertheless,
current flows — for a moment, anyway.
How can this be? When the voltage from
a source such as a battery is
connected, the negative side of the
battery voltage immediately begins to
push negative charges toward one of
the plates. Simultaneously, the positive
side of the battery voltage begins to pull
electrons (negative charges) away from
the second plate.
What permits current to flow is the
electric field that quickly builds up
between the two plates.

As the plate on
the negative side of the circuit fills with
electrons, the electric field created by
those electrons begins to push
electrons away from the plate on the
other side of the insulator, toward the
positive side of the battery voltage.
As this current flows, the negative plate
of the capacitor builds up an excess of
electrons, whereas the positive side
develops a corresponding deficiency of
electrons. Thus, voltage is developed
between the two plates of the capacitor.
But there's a catch: This current flows
only for a brief time. As the electrons
build up on the negative plate and are
depleted from the positive plate, the
voltage between the two plates
increases because the difference in
charge between the two plates
increases.
The voltage continues to increase until
the capacitor voltage equals the battery
voltage. Once the voltages are the
same, current stops flowing through the
circuit, and the capacitor is said to be
charged.
At this point the magic gets even better.

Once a capacitor has been charged, you
can disconnect the battery from the
capacitor, and the voltage will remain in
the capacitor. In other words, although
the voltage in the capacitor is created
by the battery, this voltage isn't
dependent on the battery for its
continued existence. Disconnect the
battery, and the voltage remains across
the two plates of the capacitor.
Thus, capacitors have the ability to
store charge — an ability known as
capacitance.