What are 
1)ideal voltage source 
2)non-ideal voltage source 
3)ideal current source 
4)non-ideal current source?
---------------------------------
And how do they affect a circuit?

The ideal voltage source provides a terminal voltage that is
independent of the current through it.
The ideal current source provides a current that is independent of the
voltage across its terminals.
A controlled or dependent voltage source depends on the voltage across
or current through some other element of the network.
A controlled or dependent current source provides a current whose
magnitude depends on either the voltage across or current through some
other element of the network. (this from my "Electronic Devices and
Circuits" manual when I took Electrical Engineering.)

In general circuit diagrams, unless you know otherwise, your
calculations of V=Ri are based upon "ideal" voltage and current
sources. That is to say, if the voltage source is 6v, it doesn't
change depending on R or i. A circle v (with + and - for DC current)
or i (with directional arrow) is indicative of ideal voltage or
current sources and a diamond v (with + and - for DC current) or i
(with directional arrow) is indicative of controlled voltage sources.

The controlled voltage or current sources change with their
dependencies, at a ratio indicated by, for instance, the alpha
character.

These dependencies affect the calculation of the Thevenin equivalent
voltage and impedance of (a part of) a circuit.

I don't think you'll want to use this answer as a homework answer, but
this is page 4 of the book I'm referencing, chapter "Circuit
Analysis". I think the more appropriate question would be how do
(non)ideal voltages and currents affect a particular circuit.

Hope this helps in some small way. If I recall correctly (some 12
years ago since I was in this class), this concept is one that must be
understood fairly early on in EE or else the rest of the course is
going to be quite difficult.

Regards,
crythias.

1) An ideal voltage source has zero resistance (and impedance).
2) A non-ideal voltage source has significant resistance (and/or impedance).
3) An ideal current source has infinite resistance (and zero impedance).
4) A non-ideal current source has significant resistance (and/or impedance).

You may come across the term ?hardness? of a voltage or current
source, which crudely suggests the quality of the voltage of current
source. So an engineer might say ?The current source isn?t very hard.?

An ideal voltage source will remain at exactly the same voltage
regardless of the current drawn from it. A lead acid battery is
therefore a better voltage source than a small AA cell. All voltage
sources are actually non-ideal because the ideal would have to be able
to supply infinite current. But for practical applications, voltage
sources are tailored to the required maximum current and accuracy. An
example might be a reference voltage in a thermostat circuit,
demanding perhaps as little as a few microamps. But if the current
demand is variable and the source is not very hard, one might find
that the thermostat switching temperature is not precisely repeatable.
In practice though, if the current demand is low, very hard voltage
sources are easy to design. In general, drift with temperature or time
is of far bigger concern to the designer but that is not really a
function of how ideal or non-ideal the source is, just the accuracy.

An ideal current source will supply exactly the same current
regardless of the resistance (or impedance) of the load. Current
sources are all non-ideal because the voltage of the ideal would have
to be able to rise to infinite at infinite load resistance. Again
though, for practical applications one tailors the design to match the
requirements, and when the degree of departure from perfection is
insignificant one is justified in calling the source ideal. A current
source example is the current through a CD laser which must be held to
within a few percent for correct and reliable operation. If it is too
low, the diode will not lase -- too high and it will burn out.

You probably own many voltage and current sources, usually
incorporated within chips, but both can be readily designed or bought
off the shelf. Voltage sources are far more common and range from
simple zener diodes where accuracy is not critical, to highly
stabilised voltage reference devices costing ££££$$$ which incorporate
their own oven to maintain them at a constant temperature. Current
sources however are less in demand and most are simply slightly
modified field effect transistors. They are not very hard, featuring a
resistance of a few megohms max. Bit drifty too.

So the implication of sources being not ideal is that the circuit to
which they are applied will lose accuracy. But if it?s not important,
it?s not a concern. When your car headlights are on and then you
switch on the window heater, the lights will dim slightly. Unless the
battery is really poor it won?t be a problem.

Hope this helps.

Best

Dear jack, 
          i lke to comment on ur question up to my knowledge the ideal
voltage sources are those which didnt directly depend up on the
another current source in the given circuit the voltage of the given
source is indepent of the circuit the resistance of the ideal voltage
source can taken as the infinite these are used in the dc applications
very much . if we consider the non-ideal voltage source the the
voltage corresponding to the given source is directly proportional to
the another current source and in these instruments some impedence is
present and these are more applicable in ac applications .the same
procedure is applicable for the given ideal currnet and non ideal
current source but the current source have zero resistance internally
is assumed .if u like to refer u can refer the engineering circuit
analysis by willam hayth.if u like to ask any more on eletrical ask me
please because i was an eletrical student. ok thanks for giving me
this oppurtunity