Insulators do not allow charges to pass through. They do not conduct electric charges. Air, vacuum, wood, cardboard, cotton, Helium are non conductors of electricity. They can be charged by rubbing and static electricity is generated.
When we use a material for protecting the electric charges from leaking or protecting humans or animals from electric current, we call it as an insulator.
When we describe a material or substance in terms of conductivity : we say conductor or non-conductor. But all non-conductors may not be used as insulators. Like air is a non-conductor. Perhaps we don't use it as an insulator. We may prefer some other material to air.
Semi-conductor is a non-conductor for low voltages across its terminals. When the voltage goes higher than a threshold voltage, then it conducts electricity and in fact, it gives out a higher current than that flows in to it. Thus a semi-conductor like Silicon, Germanium etc. are used in electronic gadgets and computers to control flow of current and also to amplify current or signal.