Suppose we have two metal spheres, one highly charged and the other electrically neutral. If we now place a metal object, such as a nail, so that it touches both spheres, the previously uncharged sphere quickly becomes charged. If, instead, we had connected the two spheres by a wooden rod or a piece of rubber, the uncharged ball would not become noticeably charged. Materials like the iron nail are said to be conductors of electricity, whereas wood and rubber are nonconductors or insulators.
Metals are generally good conductors, whereas most other materials are insulators (although even insulators conduct electricity very slightly). Nearly all natural materials fall into one or the other of these two very distinct categories. However, a few materials (notably silicon and germanium) fall into an intermediate category known as semiconductors.
From the atomic point of view, the electrons in an insulating material are bound very tightly to the nuclei. In a good conductor, on the other hand, some of the electrons are bound very loosely and can move about freely within the material (although they cannot leave the object easily) and are often referred to as free electrons or conduction electrons. When a positively charged object is brought close to or touches a conductor, the free electrons in the conductor are attracted by this positively charged object and move quickly toward it. On the other hand, the free electrons move swiftly away from a negatively charged object that is brought close to the conductor.