Different Motor Types
Fixed-speed motors have only one stage of output - high. This can be costly for your energy bills in the winter since every time your furnace turns on, it's dispersing the most amount of energy it possibly can.
Multi-speed motors operate much more efficiently than a single stage furnace. The first operates the majority of the time in most climates, and runs at about 65% of the furnace's full capacity. When the temperature outside becomes extremely cold, the second stage kicks on to provide the additional heat. In addition, a 2 stage furnace is much quieter since it doesn't operate at 100% capacity every time it runs and creates less carbon dioxide emissions for the environment.
Variable-speed motors have a fan motor that can move at different speeds to control the amount of heated air dispersed throughout your home. Better airflow results in a more comfortable environment and less energy consumption results in lower utility bills. They may cost more upfront, but will quickly make up for it in saved energy costs.