A flamewar is generally taken to refer to a conflict that takes place between two or more people wielding flame-throwers, matches, burning candles or other forms of incendiary device.
Within the limited and perverse context of internet discussion forums, however, a flamewar is generally taken to mean a dispute between two or more forum members, in which each types ever more angrily and abusively to the other(s), in the mistaken belief that they are the bastard children of Chuck Norris. Such exchanges are known as 'flamewars' because participants aim to metaphorically 'shoot' their opponents down in similarly metaphorical 'flames'. Participants are normally feckless, physically puny, socially inadequate individuals who've never had, and never will have, a girlfriend, since their entire life revolves around acting clever and insulting strangers on the internet.
The only thing worse than the above would be being GeoffP.