The general purpose of a conductor is to direct heat or electricity from its source to its application. The general purpose of an insulator is to constrain the heat or electricity and stop it straying outside where we want it to be.
For heat we will need a pan made of metal as they are the best conductors of heat. Aluminium, especially, is an extremely good conductor of heat. However, when the pan has hot contents and we have to lift it, the last thing we want is for the handle to be too hot to hold. So we use an insulator (plastic handle or oven glove) to stop the heat affecting our hands.
In electricity we use metal wires to conduct electricity. Copper based alloys, gold, and aluminium are all good conductors, but steel is not so good since it has a higher resistance value. Copper alloys and aluminium are mainly used, but gold is generally only used as a flashing on a conductor. The cross sectional area of the conductor will depend upon the current required to flow through it.
Various non-conducting and usually non-metallic materials are used for insulation, plastic being the most common. The nature and thickness of the insulation will depend upon the applied voltage. In the case of power lines where the voltage can be as high as 144, 000 Volts, the insulators are sinusoidal in form and made of ceramic or glass, and they support the insulator at regular intervals along its length.
Oddly, at these high voltages the current flows along the outside of the conductor and not through the centre, allowing the use of aluminium conductors with a steel core for lightness and strength.