A fanboy is any person who is the unthinking pawn of a major label/brand. A fanboy is just about the most annoying creature on earth, particularly in the heated debate of Mac vs. PC or in the realm of politics. They tend to be really irrational, and unwilling to look at both sides of the debate: repeating things by rote and being completely partisan. Also, the typical fanboy is totally uninformed and biased to a precedent, hence the repetition of rote sayings or facts/myths.

The rare form of the fangirl interested in celebrities and fictional characters as well as "geek cultures" (Doctor Who, Star Trek, etc.) If you find a fanboy you should contact the nearest fangirl immediately, as they are an endangered species.

From the noun fanboy

v. To stand, drool, or worship the object of their obsession. Fanboying involves no real physical action or movement.

One does not have to be a boy to start fanboying.

Also see fangirl

A passionate fan of various elements of geek culture (e.g. sci-fi, comics, Star Wars, video games, anime, hobbits, Magic: the Gathering, etc.), but who lets his passion override social graces.

An individual who thinks that XBox, Star Wars, comic books, action figures, TV shows, and so on are life-or-death matters that require zealotry. Such behaviors go far beyond merely being a hobby or favorite pastime. These individuals feel that social skills are not required, and see no problem with becoming hysterical over meaningless issues. They often falsely believe that they are simply showing their "inner geek", as if their behaviors are charming and a source of enjoyable humor. They also often think they are clever, and that others "just don't get it". Fanboys frequently show the characteristics of an arrogant, pseudo-intellectual, going on for hours in a droning monologue.

A way to remember conjunctions.

Someone far beyond a simple fan.
No, not the tye of fan which keeps people cool.

An arrogant person who goes into an outburst every time something he likes is questioned. Fanboys usually acuse others of being fanboys. Usually use 1337 and swarm MMORPGs. Fanboys caused alot of fallouts between people when they started arguing about consoles. If you insult something that a fanboy likes, he will spam your computer up and try to insult something that you like. Most words a fanboy uses are in 1337 or end with 'X0RZ' and they spell the work 'the' as 'teh' because they think that it is 'teh ro><0|2Z!!!'
e.g. Counterstrike is teh rockxorz!

Fanboys usually use the word 'own3d' if they beat someone.
Another annoying feature of a fanboy is that if he is loyal to a console, he usually goes to rival console forums and spams the place up with inults about how his console is 'better'.
They take the 'console war' very seriously, as if it's a real war.

(Although I'm a Nintendo fan, I hate Nintendo fanboys who can't shut up because a Sony fan said that Nintendo is for kids. Dude, chill out, the console war isn't a real war!)