A definition from the Urban Dictionary:
One who enjoys smelling farts, either self-flatulated or those originated from another man or animal. Fart huffing typically involves a series of deep and aggressive inhalation cycles, sometimes aided by a cupping motion of the palm, fanning the fart from the asshole to the nose.
Example: "I was amazed to see Jimmy huffing his dog's silent farts. He huffed as if posessed, and it made me sick to my stomach. I guess Jimmy is a real fart huffer."