Materials that make good insulators do not have the ability to conduct electricity, such as glass. They insulate because they do not have enough free atoms to conduct electricity.
In order for a material to conduct electricity it must have atoms that are loosely bound and free. When atoms are loosely bound, they are free to take on other electrons. In insulating materials, the atoms are tightly packed together, so no electrons move through them. Some materials are better at insulating than others, like glass, fused quartz and mica. Other examples of insulators include air, water and plastic, although not all of these substances offer protective functions against electricity.
Unlike insulators, conductors need free moving atoms so electrons can move from the source of electricity and through the material. One example of this is copper. Copper has loose outer electrons that can jump from one atom to another. When electricity stimulates copper, a domino effect occurs, which entails the electrons jumping from one atom to the next. As such, it acts as a good conductor. Like copper, most metals act as good conductors, because their outer valences have loose electrons. In contrast, the majority of non-metals are better at insulating, or they are poor conductors.