DC (Direct Current) variable speed or ECM (Electronically Commutated Motor) central heating or cooling system blowers
In essence, the DC variable speed or ECM blower in your central heating and/or cooling system is a much more energy efficient way of pushing the air through your home.  In single or multi-speed blowers you are left with set speeds at which the blower must run.  That means that your system has to pick one of the set speeds to operate on independent of whether or not that much power is actually needed to get the temperature of your home back to your thermostat settings.
In contrast, with DC Variable speed or ECM blowers, any multitude of blower power settings are available which means that your heating and/or cooling system can use less power more often to keep your home’s temperature constant.  An added benefit to these types of blowers is that they ramp up to speed which means they are much quieter than their set speed counterparts.