Conductors are materials that allow the flow of electric current, whereas insulators do not. While conductors and insulators are different in that respect, their similarity is in their makeup of atoms. It is these atoms that either have room to float around to facilitate conduction or have little freedom to move around; this lack of freedom makes the material they form an insulator.
While many types of conductors exist, copper is by far the best known conductor of heat, while rubber is the best known insulator in existence, according to Kid's Korner Energy Education from Florida Power & Light Company. A conductor is simply a substance in which the outer electrons of the atoms are free to move around. In sharp contrast, most insulators tightly hold on to their atoms.
Not all conductors have the same level of conductivity, just as not all insulators are resistant to electric current. While silver is one of the best conductors of heat, concrete and dirty water are also listed as heat conductors, though they are not as effective as other conductive materials such as most metals. Similarly, while glass is a good insulator at room temperatures, it becomes a conductor of heat when it is exposed to extreme temperatures.