Like any good manga, Attack on Titan soon found itself adapted into an (also good) anime… and then some light novels, and a video game, and a live-action movie, and a spin-off, and a crossover with Marvel Comics, and a… holy crap, how does all of this tie in with each other?
Well luckily for any continuity junkies out there, that dump truck full of storytelling can be roughly organized into three main canons.
The original Attack on Titan manga is currently ongoing, with 71 chapters published so far (the author predicts it will go on for another two or three years. It sets up a world where monstrous, nearly invincible giants called Titans roam the land, eating any humans they find. The remnants of humanity have taken shelter in cities surrounded by massive walls, defended by soldiers trained specifically to combat Titans using specialized gear.
The manga was a big success in Japan, leading to a prequel series that was published alongside it called Attack on Titan: Before the Fall, based on a series of light novels of the same name. Additionally, there is another manga series called Attack on Titan: No Regrets, a spin-off that follows the origin story of Levi, before he joined the Survey Corps Special Operations Squad.
All three of these are canon with each other (for the most part).
The anime TV series helped to launch Attack on Titan into mainstream popularity outside of Japan. The series has so far run for one season (with a second currently in production), accompanied by a set of five OVA episodes.
Included on the Blu-ray releases of the anime in Japan are also four visual novels, featuring side stories for some of the anime’s major characters. Finally a 16-minute drama CD featuring the cast and staff of the anime was included in an issue of Bessatsu Shōnen Magazine
All together, these form a separate continuity from the manga (although it is a fairly close adaptation overall).
Earlier this year, a live-action Attack on Titan movie was released in Japan. A second part will follow in September called Attack on Titan: End of the World.
Tying into these two movies will be a live-action TV miniseries called Attack on Titan: Beacon for Counterattack, featuring some of the same cast in a storyline about how the original Three Dimensional Maneuver Gear was created. This series of three 30-minute episodes will air on the Japanese online-video service, dTV.
Due to some big changes made while adapting the story and setting, the live-action stuff forms its own distinct continuity, separate from the anime and manga.
Considering what a cultural phenomenon Attack on Titan has been, there has been plenty of more superfluous material released around the manga/anime/live-action universes. For one, there have been a couple of video game releases (in addition to the visual novels) on a variety of platforms, none of which are explicitly canon with anything else.
There was also a short Marvel/Attack on Titan crossover comic called Attack on Avengers, which featured Spider-Man, the Guardians of the Galaxy, and some of the Avengers going toe-to-toe with a couple of Titans in New York City. For obvious reasons, this is its own thing (although I think it could make for an interesting Marvel Zombies-esque miniseries if anyone was so inclined).
On the manga side (but separate from the main continuity), there’s a series called Titan Short Skits, a yonkoma (the manga equivalent of comic strips) that was released on the Manga Box mobile app. And perhaps the most bizarre spin-off of all is yet another ongoing manga called Attack on Titan: Junior High, a comedic series set in a world where humans and Titans go to the same school. This in turn will be getting an anime series of its own, because of course it is.