In a nutshell, resistance is the ability of a circuit to resist current in a circuit (Ohms Law). Impedance adds extras (like inductance and capacitance). These are frequency dependant components. Think Loudspeakers that have both a resistive component (constant due to the copper wire) and a frequency dependant component (inductive reactance becuase of the coil in the cone). Combined together they are known as impedance at a specific frequency.
In a loud speaker at low frequencies, the resistance dominates the equation, but at higher audio frequencies the inductance comes in to account and the overall components of impedance include inductance.
Impedance (symbol Z) is a measure of the overall opposition of a
circuit to current, in other words: how much the circuit impedes
the flow of current. It is like resistance, but it also takes into
account the effects of capacitance and inductance. Impedance is
measured in ohms, symbol .
Impedance is more complex than resistance because the effects
of capacitance and inductance vary with the frequency of the
current passing through the circuit and this means impedance
varies with frequency! The effect of resistance is constant
regardless of frequency.
Yusuf, you have posted several 'what is....' questions. It would be quicker for you to just Google for answers and you would get a wider set of definitions.
Well if heaps of people are trying to get through a door into a nightclub, and there is a big bouncer at the door that only lets a few people through each hour...