Well otaku just means obsessed, or something along those lines and is used as sort of a "geek" term, generally otaku are associated with anime, but there's plenty of varieties of otaku.

Well I know a few japanese Otaku that do not watch anime, they are military otakus and not that rare. The term otaku is used for anyone 'obsessed' with his hobby or certain thing, internationally it only found it's use when it came to anime.