Insulators are parts of an atom which hold in the electrons very tightly and don't permit them to move freely from one atom to the next. Examples of insulators are plastic, cloth, grass and dry air. Conductors hold atoms very loosely and allow the electrons to move through them quite easily. Most metals are good conductors.
Even though it is quite easy for electrons to move through conductors, electrons need to be provoked in order to move from one atom to the next. An atom needs to touch another atom in order to move electrons. Rubbing is not necessary, although it makes the atoms touch at many different points, thus increasing the chances that electrons will be transferred.
