The laws of thermodynamics tell us that when a mass is hotter or cooler than what surrounds it, heat transfer will take place in a natural attempt to even out the differences.  When that mass is hotter or cooler by our effort, this will result in a waste of energy.  It is our task here to prevent this undesirable energy exchange. 

Insulators are substances which resist the transfer of heat by blocking the flow through conduction, convection or radiation.  Some insulators handle one of these avenues of un wanted heat loss or gain.  Some handle more than one.