A material that does not conduct electricity well is an insulator.
In an electrical insulator, charge carriers cannot move freely, so electric current is strongly resisted. Common examples include glass, rubber, plastic, paper, porcelain, and PTFE. Insulators are used to surround wires and to support high-voltage conductors while preventing unwanted current from reaching people, equipment, or the ground.
The distinction is not absolute. No real material is a perfect insulator, and even a good insulator can conduct when the applied voltage becomes high enough. This failure is called electrical breakdown; the voltage at which it occurs is the breakdown voltage. Heat, moisture, impurities, and the material’s thickness can also affect its insulating performance.
Insulators differ from conductors, whose mobile electrons carry current readily, and semiconductors, whose conductivity can be controlled by factors such as temperature, impurities, or applied voltage. A superconductor is different again: below a critical temperature, it can carry current with essentially zero electrical resistance.