From MetaFilter:
"This phenomenon has been recognized since the earliest days of the Internet (then the Arpanet, used by a small circle of scientists) as "flaming," the tendency to send abrasive, angry or otherwise emotionally "off" cyber-messages. The hallmark of a flame is that the same person would never say the words in the email to the recipient were they face-to-face. His inhibitory circuits would not allow it — and so the interaction would go more smoothly. He might still communicate the same core information face-to-face, but in a more skillful manner. Offline and in life, people who flame repeatedly tend to become friendless, or get fired (unless they already run the company)."