Hackers are classified as a human threat and not a classification by itself.
All the other answers are incorrect. Threats result from a variety of factors, although they are classified in three types: Natural (e.g., hurricane, tornado, flood and fire), human (e.g. operator error, sabotage, malicious code) or technological (e.g. equipment failure, software error, telecommunications network outage, electric power failure).
Reference: SWANSON, Marianne, & al., National Institute of Standards and Technology (NIST), http://csrc.nist.gov/publications/ nistpubs/800-34-rev1/sp800-34-rev1_errata-Nov11-2010.pdf, June 2002 (page 6).