To send nasty or insulting messages via e-mail or to post them on a newsgroup or a blog. This is usually done in response to someone having broken the rules of netiquette. A "flamer" is someone who sends these message, sometimes resulting in a flame war.
Read the New York Times article below "Flame First, Think Later: New Clues to E-Mail Misbehavior"
NetLingo Classification: Online Jargon