Otaku actually expresses your love or fetish (non sexual) for a certain thing. So you can be an anime otaku, movie otaku, metal otaku, sport otaku, cook otaku etc. Though it has taken a bad meaning through the years because of the kind of people that have those obsessive love about things, so it can be considered bad in japan as bad as if you called someone a geek or nerd in US. Look here for more info: http://en.wikipedia.org/wiki/Otaku
I agree with the person in post #2. You have to decide yourself if you are an otaku or not. But be careful it's not too flattering to have that name on you. Not only because of what other people will think but also for yourself. I mean that if you just decide to call yourself an otaku you may start subconciously doing things that will affect your everyday life, sometimes to the worse.