Electrons (ie. electricity) can move through some materials much more easily than others. The wires connecting the mains electricity to a mains appliance is normally made of copper as this allows the electrons to pass very easily, but to save you getting electrocuted every time you touch the power lead the copper wire is covered in a plastic coating which does not allow the electrons to pass through.

Materials that allow the electrons to move easily are called conductors, whereas those that prevent electrons from flowing are called insulators. It is these properties that allow us to control where electricity is allowed to pass and to be able to turn devices on and off. The insulating properties of a material will differ depending upon the material and the thickness, so an appropriate insulator should always be used when dealing with electricity especially with mains electricity