The resistor is a device that is common to practically every circuit known to man. Resistors are designed to have specific values of resistance. Resistance opposes the flow of electrons. A resistor is a device that limits the amount of current flowing through a circuit for a particular applied voltage. That is to say... only so much current will flow with a certain amount of voltage and resistance. For example...
The basic unit of resistance is the ohm. 1 Ohm of resistance is defined as ... the resistance of a circuit in which a 1-amp current flows when 1 volt is applied.
One of the most fascinating things about electricity is how
it can be used to create a magnetic field. 
Electromagnets
were used in telegraph relays for years before the advent of radio. Coils are
still used today in relays, solenoids, transformers, and inductors. Basically,
current flows through a coil of wires and the energy from the current flow produces
a magnetic field. This ability to store electrical energy in a magnetic field
is called inductance.
The inductance of a coil would change depending on the number of turns of wire, the length of the coil, and the diameter of the coil. There is a basic unit for inductance ... the Henry (abbreviated "H").
One application of the inductor is to limit the flow of alternating current while allowing direct current to pass freely.
SIDE NOTE: Allow me to digress for a moment if you
please. There is a good way to imagine how an inductor works in the water
analogy from lesson A. If
you were to hook up a flywheel connected to a turbine that was in line with
your water flow... 
that
would be a good description of an inductor and how it works with electricity.
If the water was flowing constantly in one direction, it takes a little while
for the turbine and flywheel to get up to speed ... and the flywheel resists
the initial flow of current as it starts to gain momentum... but, once the
flywheel gets up to speed, there is no more resistance to the current flow
... so long as the current stays at the same speed.
Now... if your current alternates back and forth (now... I know that water doesn't REALLY flow in alternating current, but bear with me for a moment) then the flywheel/turbine takes some time to build up to the initial direction that the current wants to flow... and about the time that the current switches back the other way, the flywheel is trying to push the first way it was going ... it creates a lot of extra "resistance" to the change in current flow. It takes it a while, but eventually the flywheel/turbine device will start back the other way. This is how an inductor will react to alternating current. It really impedes the flow of current when it is alternating... but if it is direct current, then there is very little resistance to the flow.
Another way that electrical energy can be stored is in an electric field. A good example of an electric field is static electricity and static buildup. Many of you that live in the North where it gets very dry in the cold winter months have experienced the sensation of static electricity when shaking hands with one another, or touching a door knob, refridgerator handle, or light switch. This is electrical energy that has been built up and stored in you (and your clothing). Another example is seen in lightning. The clouds build up a static charge that is exhibited in great bolts of static electricity leaping from cloud to cloud, or cloud to ground. The capacitor is merely a device that will store certain amounts of static electricity ... creating what is known as an electric field. Capacitance is the ability to store an electric field. The basic unit of capacitance is the Farad (abbreviated "F").
In a capacitor, there are two electric plates separated by
an insulating material (plastic, glass, air...). 
These
two plates (or in some cases... series of plates - large capacitors often have
many plates hooked up together to increase the capacitance) are hooked up to
two leads that allow the current to flow in and out of the capacitor. As the
current flows, electrons build up on one plate. At the same time, electrons
flow out of the other plate. Eventually, the capacitor is completely "charged
up" and no more current will flow. There is a positive charge on one plate
and a negative charge on the other plate. No more current will flow, because
the voltage is not able to charge the plates up any higher.
Now if the current was only a direct current, a capacitor would eventually become charged up and no more current would flow through the capacitor. BUT, if the current was an alternating current, and the current switched directions backwards, then the capacitor would start to "uncharge" as the current began to flow the opposite direction.
One application of capacitors in electronic circuits is to block the flow of direct current while allowing alternating current to pass.
SIDE NOTE: Another way to picture the way a capacitor
works with the water model (lesson
A) is to imagine a very stretcy rubber balloon. Now take this balloon
and stick it in between two sections of pipe where the water flows. 
As
the water starts to flow one direction, it pushes against the balloon and
causes it to stretch. This pushes the water on the other side ... allowing
the water to flow "through" the balloon. As the balloon stretches
far enough, it begins to provide enough force to press against the water pressure
that was causing it to stretch. The current no longer is able to stretch the
balloon, and so the current stops flowing. This is like when a capacitor is
completely charged. This is why a capacitor does not allow direct current
to flow.
Now if the current is alternating back and forth, then the balloon will begin to stretch with the current flow, and then as the current switches and goes the opposite direction, the balloon begins to unstretch and then go back the other direction until it stops stretching or the current begins to switch back again. This is like a capacitor hooked up to alternating current. A capacitor will allow alternating current to flow "through" it. (I say "through it", but you can clearly see in this illustration that the current never really flows through the baloon... and in real life, the current never really flows through a capacitor either.)
An additional note... just as a certain amount amount of pressure will cause the balloon to break, there is a certain amount of voltage that will cause the capacitor to short out - current will jump across the two plates. This may damage the insulating material in between the plates (if the material is something other than air) and reduce the capacity of the capacitor.