Basically, the NEC divides Hazardous Atmospheres into the following (courtesy of the gelightingsystems.com website, since I don't have my Code book handy):
CLASS I locations are those in which flammable Gases or Vapors are ormay be present in the air in quantities sufficient to produce explosive orignitable mixtures.
CLASS II locations are those that are hazardous because of the presence of Combustible Dust.
CLASS III locations are those that are hazardous because of the presence of easily ignitable Fibers or Filings, but in which such fibers or filings are not likely to be in suspension in the air in quantities sufficient to produce ignitable mixtures
Each “CLASS” is further defined as either Division 1 or Division 2.
DIVISION 1 is an environment that is "Normally Hazardous".
DIVISION 2 is an environment that is "Not Normally Hazardous".
