In modern Japanese slang, the word otaku refers to anyone who is fan of a particular hobby or interest, with varying degrees of obsession. In Japan, otakus can be very specialized in their interest, from cosplay otaku (fans of dressing up in costume) to gemu otaku (video game fans). The English language has adopted the term to refer to a person who is a fan of Japanese animation specifically or a fan of Japanese culture in general.
While the term still carries negative connotations in Japan (often meaning a creepy person, who rarely leaves home and has an unhealthy obsession), it is a term that many in English-speaking countries carry with pride. It’s very similar to the English term geek, which for many years was an insult, but with the advent of modern technology is now a fairly neutral term.
In the use, nerd culture has reached a peak of acceptance. With the popularity of the Marvel Cinematic Universe, shows like Game of Thrones and the Big Bang Theory and the increasing inclusiveness of the modern comic book to include empowered women, LGBTQ characters and minorities, more and more people are able to now call themselves nerds, geeks and otaku.
In the same way that if you eat several pieces of raw fish on rice, you’re eating sushi, not sushis, note that otaku can be both singular and plural in its usage. A singular anime fan can be an otaku, and a group of anime fans can be called otaku.