The NYT has a great article today on flaming. I still view the shirky article "Group as User: Flaming and the Design of Social Software" as the best explanation of online flaming, but the new article is perhaps easier to read. From the article:
Flame First, Think Later: New Clues to E-Mail Misbehavior
…flaming, or sending a message that is taken as offensive, embarrassing or downright rude.
The hallmark of the flame is precisely what Jett lamented: thoughts expressed while sitting alone at the keyboard would be put more diplomatically — or go unmentioned — face to face.
Flaming has a technical name, the “online disinhibition effect,†which psychologists apply to the many ways people behave with less restraint in cyberspace.
(Ed: link added by me).