An extreme fan or follower of a particular medium or concept, whether it be sports, television, film directors, video games (the most common usage), etc.
Known for a complete lack of objectivity in relation to their preferred focus. Usually argue with circular logic that they refuse to acknowledge. Arguments or debates with such are usually futile. Every flaw is spun into semi-virtues and everything else, blown to comedic, complimentary proportions.
Known for using the phrase
"Object of affection = Best Ever"
However, while people only really say that as hyperbole, fanboys truly believe it.
Troll the internet to spread the gospel.
Insult/chastise others for using public forums to express an objective opinion, no matter how constructive or, respectful it may be. (Go ahead and admit that it's a good game/movie/etc. This fact and anything else will be promptly ignored in favor of cherry picking the negative, and beating you over the head with it.
Tend to resort to petty annoyance replies when backed against the wall. Usually grammar attacks and non-replies.
"Final Fantasy VII is the best RPG ever!"
"Ocarina of Time is the best game ever!"
"Linux is the best OS ever!"
"Xbox is the best console ever!"
"Dragonball Z is the best show ever!"
The sad thing is, fanboys don't realize, that these companies that fuel their passion, don't really give a damn about us. To me, defending a multi-national corporation like their your own family, is ridiculous. These organizations don't come to your house and help you when you're sick, they don't help you move, or drive you to the airport. They don't support you in any way. It's a business, not a friendship. As soon as you can no longer buy their product, you disappear to them.
Prices shown in USD.