Simply, otaku is someone who's maniac of fanatic for something. So you can have any kind of otaku. As a concrete example, take your national sport, try to think of the most dedicated fan of that sport and you have the perfect image of your national sport's otaku (it's a somewhat negative term). Otaku, in a different sense (different writing), can also mean "your home" or "your house", but I have a feeling that's not what you're hinting at...
Now, I don't know the slant it's taken in Blegium, but in North American, the term otaku is proudly worn by manga and anime fans. I used to refer to myself as an otaku, but since coming to Japan, I've abandoned the habit...
my mum had asked me what otaku meant and i told her it meant 'geek enthusiast', i always think of giggling trekkies when i think of otaku. i think 'fanatic' may be a better thing to tell people from now on.
No special connection with anime/manga except in the mind of North Americans, as far as I know. Cuz, in Japan, the term otaku is never used by itself. It is always preceded by the theme of the obsession. E.g. anime otaku, baiku (motorcyle) otaku, kuruma (car) otaku, eiga (movie) otaku, etc., ad nauseam.
In other words
In Japan, otaku=enthusiast (pushed to the point of fanatism)
In North American (perhaps Europe too, I don't know), otaku=manga/anime fan, collector, enthusiast. So basically, otaku are to manga/anime, what Trekkies are to Star Trek.