1. (This is
how it is used in
Japan) Otaku means a person who sits alone and does nothing all day day and might watch anime, read manga, ect. In Japan, it is a term that is not taken as a
complement.
2. (This is used in other places) An Otaku is a person who is obsessed in a fandom; usually Japanese. In other places (not Japan), it is not taken offensively.