I think of it in a tiered structure:
Nerd
....Geek
........Otaku
Where a nerd is a person who has more interest in [subject] than most people who have any interest at all in [subject], a geek is someone who has an "unhealthy" interest in [subject] to the point of ignoring other things (like a social life), and an otaku is someone who takes obsession with [subject] to a new level to the point of being disinterested in anything but [subject].
For example: I am a linguistics nerd. Borderline geek, but it balances out through my keeping my head in my math book. I used to be a military air tech otaku because all I could talk about was F-16 this, F-22 that, latest-radar-guidance-system everything-else, and it occupied a too-large portion of my through process from day to day.
F-22 Raptor pwns all, btw
. (SEE?!)
Anyway... hope that all made sense. ^_^ v