From Social Patterns
In the context of managing social networks and information in the public domain, Norms refer to a pattern, an expected (as contrasted to established by law) behaviors when operating or working with a system.
Norms are socially-enforced, and less restrictive than 'rules', although this does not quite reflect on the actual effectiveness of a Norm as compared to a legally-established Rule. In many cases, Norms appear to be more effective in moulding or directing user behavior as compared to rules because of greater visibility of actions in a broader community. This appears to align with the fundamental observation that a broad community involvement in management is far better than the management entrusted to a select few.
See also Gary Burnett's essay on Explicit and implicit norms in online groups.