Resistance is a real part of Impedance. Basically Impedance 
has two parts a)Resitance b)Reactance in case of 
alternating current but in case of direct current it has 
only one part that is resistance.We can understand it by 
the following eqution:

                Z=R+jX (all quantities in vector form) 
Where the real part of impedance is R it means voltage and 
current are in phase and the imaginary part is reactance X. 
Impedance is also define by the equation  Z=V/I (all 
quantities in vector form) 
Reactance occurs due to inductance or capacitance of a 
circuit so it has also two types a)capacitive reactance b)
inductive reactance given by the equation, X=ZsinQ.In case 
of Inductive reactance the current is 90 deg. lag with the 
voltage and 90 deg lead in case of capacitive reactance in 
Idea condition. We can also define impedace by impedance 
diagram and find out the  relation between resistance 
reactance & impedance another equation of impedance is 
Z=RcosQ where cosQ is the power factor of the system

In A.C. supply reactance comes in to picture and in D.C. 
supply there is a resistance.

         z = R + jX