Fanboys are immature fans similar to fangirls... but beyond the gender difference, they sometimes have vastly different approaches to mutilating their fandom of choice. Many are prone to rage and cases of fandom elitism. Fanboys usually are more solitary creatures — idolizing, defending, and drooling over their fandom to the point of insulting or even fighting with other people. They are not as prone to stampeding, but should still be treated in the same way. Like the fangirl, the fanboy also shows zero maturity or emotional restraint, often by being patronizing to other fans and being amazingly perverted towards their lust object.
A fanboy will often learn every aspect of whatever he is a fan of to the most minute details, and exhibit disdain of anything that is not what he is a fan of. This includes things inside the fandom itself — such as the 'real' way to play a video game, or the 'real' meaning of a TV show or anime that of course only they and a few others see... and they're vocal about it. Very, and annoyingly vocal.
Fanboys tend to be part of flame wars.
Note also that calling an Agent a fanboy may cause cases of head trauma and missing limbs.