When being called Otaku basically means that you are obsessed with something but it's mais known for Anime/Manga/Videogame fans. Believe it or not being called Otaku in Japão is actually an insult (just in case any of you were thinking of traveling to Japan, there's a little tip). I, for one, like being called Otaku (then again I live in the U.S.) because it reminds me of Konata Izumi from the animê "Lucky Star" and we're a lot alike. :3
