Otaku and Anoraks Geeks* in America are sometimes lacking the terminology to describe themselves, and can sometimes be left with terms that seem a little bland. "Fanboy," a word often used to describe someone who really really loves comics or movies or video games, seems a little weak, and, to be honest, belittling. Right off the gate if you describe yourself as a fanboy you seem to be admitting weakness and immaturity, which is not necessarily what you're going for if you want to convey to someone that you know everything there is to know about Aquaman and can speak three different dialects of Elvish without an accent.
* As opposed to nerds and dorks, that is. This is a distinction I really should go into with more detail at some point in the future, since it's something I explain to people a lot and I think is useful and everyone should adopt. Simply put: Geeks are media obsessed, nerds are bookish, and dorks are just awkward.
And maybe you should feel weak and immature for knowing these things--I certainly go back and forth between insulting fanboyish knowledge (when it comes off like Comic Book Guy from The Simpsons) and defending fanboyish knowledge when it happens to be something I know a lot about (the Physics of space combat involving faster than light travel**, maybe). There's a reason people who speak Klingon aren't invited to all the cool parties, but I can't rightly say right now whether it's a good reason.
** A post I am working on currently and, I hope, going to share shortly, because it's fascinating.
The Japanese have a better word for it, though: otaku. The word itself apparently comes from the Japanese word for "another's house or family." So, your home/family away from home. In Japanese, the word is used for someone who has an obsessive interest in manga, anime, teen idols, or video games. It seems to have mostly started out as a slur, though it seems to have been at least partially reclaimed as a badge of honor--the reclaiming being fraught with complexity in a way comparable, but lesser in degree, to words such as "nigger" or "fag" have been for their respective people.
Here in the US it's used with less complexity, mostly because the only people who even know the word otaku are probably the ones applying it to themselves (it also seems to mostly be used to refer to people who have an obsessive interest in Japanese pop culture: you can be an anime otaku over here, but not usually a Buffy the Vampire Slayer otaku).
The British also have a word for this, apparently. British otaku are known as anoraks. The best guess for the origin of that comes from the anoraks trainspotters used while engaging in their hobby. The entire thing has expanded to mean people who have obsessive interests in niche subjects (note that trainspotters are not geeks in the sense of otaku. That seems to indicate to me that anoraks can be geeky or nerdy. It's the interest in something that isolates them from society as a whole that seems to be the defining characteristics).
Since anoraks (the clothing) aren't as prevalent here, but do actually exist in English, it's understandable that the loanword otaku might be more likely to spread over here rather than anorak. Plus, it's a truth universally acknowledged amongst geeks that Japan is awesome, while England is not (nerds seem to be split 50/50 on this, and some of those nerds might gloat that the sentence this parenthetical is a parenthetical of is a paraphrase of Jane Austen. England FTW!). The alienness of the word makes it more useful to introduce into the culture.
The great thing about the Internet is that people can feel less alone and weird, even if they're an anorak, otaku, or just plain fanboy geek. Though maybe obsessing over things on the internet isn't all that healthy, it's healthier than crushing loneliness.