Google - (in Japan) an Otaku is a young person who is obsessed with computers or particular aspects of popular culture to the detriment of their social skills.  In Japanese, Otaku literally means "in house" eluding to the idea that these people are reluctant to leaving the house.  

Although the obsession of an Otaku can be pretty much anything, these days it mostly relates to those obsessed with Animé or Manga. 

Studio Gainax's "Otaku no Video 1985,"