Otaku - it's either a proud banner you wave around for everyone to see or a guilty secret that you keep to yourself. Otaku refers to a person with a passionately obsessive interest in anything. The correct usage of the term would be to place the word referring to what the person is obsessed about before the word otaku (e.g., anime otaku or manga otaku). Most people think that this term refers exclusively to anime addicts and manga addicts. But in reality, otaku can refer to anything, from anime to the military.
There are some common types of otaku in Japan and in the world, and these are the anime otaku, the manga otaku, the pasokon otaku, the gemu otaku, the wota, and the gunji otaku.
Anime Otaku are people who practically live, breathe, and eat anime. They watch anime whenever they can, and think about anime whenever they can't. They are the die-hard fans of anime series. A common example would be those people who wear hitai-ate's or konoha forehead protectors in pubic for no particular reason. They're the ones who idolize Naruto so much that they add 'dattebayo' to the end of every sentence. They're the ones who wear schoolgirl uniforms even when it's not a school day. They're the ones who know all the information about their favorite series and can recall even the minutest detail at any given time.
Oftentimes, anime otaku are also manga otaku, as anime shows are usually derived from manga. And therefore those who watch anime usually read the manga too. Manga refers to the Japanese comic books. Manga Otaku are the ones who can be found waiting for the bookstores to open early in the morning so that they can get first dibs on the latest releases of their favorite manga. The term can also be extended to the obsession in doujinshis. A Doujin (short for doujinshi) is a fan made comic book, an unofficial spin-off from the official manga, more often than not portraying the life of a character. They're released by a bunch of doujin circles, talented artists who devote their time making these doujinshi.
Pasokon otaku are people who are obsessed with their personal computers. They're like tech-geeks, always itching to get their hands on the latest gadgets and gizmos for their computers. They're so knowledgeable about computers that they would probably be accepted into Microsoft without having to bat an eyelash. They usually own their own weblogs and their own websites and might even know how to make their own games. It's also very rare to find a pasokon with 20/20 vision.
Gemu otakus are those who are obsessed with video games of any genre. Roleplaying games, MMORPG's (massively multiplayer online games), action games, shoot 'em up games, platform games, puzzle games, dance revolution, guitar hero, strategy games, racing games, sports games, dating games... you name it, they play it.
Wota are those who are addicted to their 'idols'. They're addicted to pop icons, pretty pop princesses, boy bands, etc.
Gunji otaku are those who are obsessed with anything related to the military such as guns, strategies, tanks, planes and helicopters.
There are still tons of other types of Otakus, but the main idea here is obsession onto a certain hobby or things. That's pretty much it on the explanation in the term Otaku in a nutshell, hopefully you've learned something new about it.