In modern Japanese slang, the term otaku refers to a fan of any particular theme, topic, or hobby. Common uses are anime otaku (a fan of anime), cosplay otaku and manga otaku (a fan of Japanese comic books), pasokon otaku (personal computer geeks), geemu otaku (playing video games), and wota (pronounced 'ota', previously referred to as "idol otaku") that are extreme fans of idols, heavily promoted singing girls. There are also tetsudou otaku or denshamania (railfans) or gunji otaku (military geeks).
While these are the most common uses, the word can be applied to anything (music otaku, martial arts otaku, cooking otaku, etc).
The loan-words maniakku or mania (from the English "maniac" and "mania") are sometimes used in relation to specialist hobbies and interests. They can indicate someone with otaku leanings. For example, Gundam Mania would describe a person who is very interested in the anime series Gundam. They can also describe the focus of such interests (a maniakku geemu would be a particularly underground or eccentric game appealing primarily to otaku). The nuance of maniakku in Japanese is softer and less likely to cause offense than otaku.
Some of Japan's otaku use the term to describe themselves and their friends semi-humorously, accepting their position as fans, and some even use the term proudly, attempting to reclaim it from its negative connotations. In general colloquial usage however, most Japanese would consider it undesirable to be described in a serious fashion as "otaku"; many even consider it to be a genuine insult.
An interesting modern look into the otaku culture has surfaced with an allegedly true story surfacing on the largest internet bulletin board 2channel: "Densha Otoko" or "Train Man", a love story about a geek and a beautiful woman who meet on a train. The story has enjoyed a compilation in novel form, several comic book adaptations, a movie released in June 2005, a theme song Love Parade for this movie by a popular Japanese band named Orange Range and a television series that aired on Fuji TV from June to September 2005. The drama has become another hot topic in Japan, and the novel, film and television series give a closer look into the otaku culture. In Japan its popularity and positive portrayal of the main character has helped to reduce negative stereotypes about otaku, and increase the acceptability of some otaku hobbies.
A subset of otaku are the Akiba-kei, men who spend a lot of time in Akihabara in Tokyo and who are mainly obsessive about anime, idols and games. Sometimes the term is used to describe something pertaining to the subculture that surrounds anime, idols and games in Japan. This subculture places an emphasis on certain services and has its own system for judgment of anime, dating simulations and/or role-playing games and some manga (often doujinshi) based upon the level of fanservice in the work. Another popular criterion — how ideal the female protagonist of the show is — is often characterized by a level of stylized cuteness and child-like behavior. In addition, this subculture places great emphasis on knowledge of individual key animators and directors and of minute details within works. The international subculture is influenced by the Japanese one, but differs in many areas often based upon region.
On the matter, in recent years "idol otaku" are naming themselves simply as Wota (ヲタ) as a way to differentiate from traditional otaku. The word was derived by dropping the last mora, leaving ota (オタ) and then replacing o (オ) with the identically sounding character wo (ヲ), leaving the pronunciation unchanged.
In Japan, anime is not as widely accepted and mainstream as manga. Because of this the otaku subculture has much influence over the mainstream anime industry in Japan. The area where otaku have the most influence in manga tends to be with doujinshi. Manga published in the United States are more influenced by their respective otaku subculture than they are in Japan. This is because most people who read manga have some ties to the subculture in the US, whereas in Japan manga reading is more widespread.