You can help everyone by it.
Otaku is a term used in Japan to talk about people with obsessive interests. While it is usually though to mean only anime addicts, it can also refer to all sorts of otaku. Some examples being train otaku, car otaku or robot otaku. Examples of many of these types can be found in Densha Otoko. Lots of otaku post on 2ch or Futaba Channel. Lots of otaku visit Akihabara.
Usage of the word "Otaku" by English-speaking fans
While the word was never particularly complimentary, Japan in the late eighties went through a phase of "Otaku panic" caused by a particularly murdersome anime-obssesed lolicon, causing many Japanese to assume that all otaku were cannibalistic paedophile child-murderers. Strangely enough, this period of the term being lower than mud was just around the time when anime was really beginning to take off in America; consequently the somewhat amusing phenomenon of western anime fans declaring themselves "the biggest Otaku in Shitkicker, Illinois" developed. Even stranger however, is that as awareness of this fact was slowly spreading through the western fandom, the word "Otaku" was undergoing something of a rehabilitation back in Japan as a result of the aforementioned "Densha Otoko" phenomenon. The More You Know. Also, your mom's a fag.