OTAKU

(1) Original meaning: Your house (company, organiztion, etc), used as a polite form of the second person => you

(2) Meaning in early-late 1980's: An extreme fan of anime/manga/sf who lacks communications with other people and usually untidy => nerd, fanboy [Otakus used "otaku" for "you" instead of more common "anata" "anta" and that' what gave them the title of Otaku-Zoku (otaku-race)]

(3) Current Usage: Anyone obssesed or overly interested with any subject <ex.> Car otaku, Gun Otaku, etc... => mania, freak

The second has a VERY negative meaning and the third still carries negative meaning, unless used between otakus, of course. So use "otaku" to only other anime fans.

On the net, Otaku is usually refered to a big fan of anime and/or manga.