These are materials in which it is easy to get electrons to move and provide a flow of electric current.
Conductors are mostly metals such as gold, silver, copper, iron and lead.
Carbon is a conductor as well as some gases (as in fluorescent tubes) and water containing some chemicals.
These are not perfect conductors and offer some resistance to the flow of current.

The resistance of a conductor (such as a metal rod) is determined by three things.

(1) its length. The longer its length the higher its resistance.
(2) its cross-sectional area. The bigger this is the lower is its resistance.
(3) the material of which it is made. 

All materials have RESISTIVITY.
The higher the value of resistivity the higher the resistance.
It is measured in OHM METRES.

                                length x resistivity
 Resistance  =     -------------------------------
                               cross-sectional area
 

These are materials in which it is difficult to get current to flow.
Examples are rubber, pvc, paper, polystyrene and oil.
Even with these it is possible to get some current flowing if the applied voltage is high enough.

There is another class of materials called semi-conductors.
These have a resistance between insulators and conductors.
Examples are silicon and germanium and are used in diodes and transistors.