Insulators are materials which prevent the flow of heat (thermal insulators) or electric charge (electrical insulators). The opposite of electrical insulators are conductors and semiconductors, which permit the flow of charge (Note: a semiconductor is strictly speaking also an insulator, since it prevents the flow of electric charge at low temperatures, unless it is doped with atoms that release extra charges to carry the current). An electrical insulator may sometimes be known as a dielectric.
A perfect insulator is impossible to achieve due to the second law of thermodynamics. However, some materials (such as silicon dioxide) are very nearly perfect electrical insulators, which allows flash memory technology. A much larger class of materials, e.g. rubber and many plastics, are "good enough" insulators to be used for home and office wiring (into the hundreds of volts) without noticeable loss of safety or efficiency.
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