The Web Application Security Consortium has released a paper (PDF link) on threat classification. Its intention is to clarify and organize the threats to the security of a Web site. The goals of this project:

• Identify all known web application security classes of attack.
• Agree on naming for each class of attack.
• Develop a structured manner to organize the classes of attack.
• Develop documentation that provides generic descriptions of each class of attack.

Definitely an interesting read if you are concerned about Web site security.