Otaku is a Japanese term used to refer to people with obsessive interests, particularly anime, manga or video games. In modern Japanese slang, the term otaku is most often equivalent to "geek". However, it can relate to a fan of any particular theme, topic, hobby or any form of entertainment. In English, it is usually used to refer to an obsessive fan of anime/manga and/or Japanese culture generally, and to a lesser extent Japanese video games.

Source:

http://en.wikipedia.org/wiki/Otaku