• "Huffing" involves breathing through an inhalant-soaked rag held over or stuffed into the mouth; or
  Inhaling from balloons filled with nitrous oxide.
• "Sniffing" or "Snorting" implies inhaling vapors from a container.
• "Spraying" implies spraying an aerosol directly into the mouth.
• "Bagging" refers to sniffing or inhaling fumes from substances sprayed or deposited inside a plastic or paper bag.
  

Source: http://www.usdoj.gov/ndic/pubs07/708/index.htm#How