A person who spends a considerable amount of time on internet forums dedicated to specific interests (music, games, etc). Consider themselves experts on said specific interest while (usually) not contributing anything to the interest themselves. A good size of their input, often the majority of it, is negative; they reason that this is the easiest way to prove themselves knowledgeable on the subject. They have little clout in real life regarding whatever topic the forum is dedicated to, so the forum is the only way they can feel they are contributing.
Person 1: Why does John spend hours each day reviewing songs and albums on the rock music forum, calling most of what he reviews terrible, when he doesn't play an instrument or contribute to music himself?
Person 2: Because he's a stupid forum warrior, that's why.