Dirty sock syndrome is the foul, moldy smelling odor that comes from your air conditioner when you first turn it on.  It is caused by the build up of mold and bacteria on your evaporator coil.  Mold and mildew can grow due to the moisture that builds up on your coil through continued use, is fueled by the dust that collects in your air conditioner over time from inadequate filtration and is especially common when your unit goes from a period of heating to cooling.