My term for an Otaku is someone who has a large collection of anime/manga, and wants to go in-depth with it. This can be making drawings, researching the culture behind the anime, collecting merchandise etc.
The differences in fandom come like this:
Fair-weather fan - has the odd book/series. They're just bored and thought the book/show was interesting.
Anime/Manga fan - Has a number of manga and anime series, but they collect a lot of Hollywood/Western stuff too. They may have the odd merchandise, but no collection.
Otaku - The dedicated. They will get shirts, models and have a large collection. They will try to get as much fun out of anime/manga as possible, as they are passionate about it (for better or worse). This doesn't necessary mean they're pervs or psychos, although there are some wierdos that go to any length.
I've been called an otaku by another anime fan, so I consider that to be (and proud of it.) While Ryo collects lots of anime, I'm more the manga side of the expenses. I do have my own series though, so I consider it a team effort.
I'm no spokesperson, but I dont mind socialising. I just dont get invited most places (usually just workmates out for the end-of-month clubbing). I don't have much gossip knowledge, which is what 90% of chat is. I just reply graciously and try to be a good guest.