My own personal definition, from the dicitionary of Angelo Credo.
Fanboy: Adjective: A word used to describe those who are so very obsessed with a given film, band, game, gaming platform and the like, lately most applied to the latter of these, to the point where it gets worrying, there is no known reason for the level of sheer stupidty they exibit; seemingly unable to accept real logic and fact if it so happens to either A: Prove them wrong, or B: Make what they love look bad, they will deny, with great bravado and gusto, even the most factual and flawless arguments that no one in their right mind would be able to pick apart in order to defend their precious idol.
Not a single "hardcore" fanboy is in the correct frame of mind, there is no real reason for the way that they are, and no one but them could offer any logical insight into their ways, but then again, that would require you to be talking to a fanboy that grasped the concept of logic to begin with, and those fanboys are absolutely non-existant.
Don't try to understand them, just don't, honestly, not only is it more effort than it's worth, but you'll get a headache just trying, it's like trying to understand the Grandfather Paradox relating to the theory of time travel, it'll make your brain explode just trying.
EDIT: If you ever encounter a being of such immense stupidity, the first and immediate course of action is to eliminate them, at whatever the cost, doing so will raise the global IQ standing by at least 10 points.