Ajin is originally a manga, that has subsequently had a few adaptations into anime. In November 2015, it seems to have been adapted into a movie: Ajin Part 1: Shoudou (Ajin: Demi-Human - Compel). MAL's blurb for the movie reads:
For high schooler Kei—and for at least forty-six others—immortality comes as the nastiest surprise ever.
Sadly for Kei, such a feat doesn't make him a superhero. In the eyes of both the general public and governments, he's a rare specimen who needs to be hunted down and handed over to scientists to be experimented on for life—a demi-human who must die a thousand deaths for the benefit of humanity.
The movie lists two yet unreleased sequels: Ajin Part 2: Shoutotsu, set to be released in May 2016; and Ajin Part 3: Shougeki, set to be released in Seprember 2016. The blurb for these two is the same as the one for the first movie.
The manga was also adapted into an anime series, Ajin, released between January and April 2016 (the Winter 2016 season). This one lists the three movies as "Alternative versions", and its blurb reads:
Where there is life, there is death... or is there? What would you do if you could never die? The possibilities are endless.
Seventeen years ago, the existence of a special species called Ajin was discovered in Africa. They are said to be immortal beings, with several supernatural abilities, that look just like any other human being. Are these Ajin just random phenomena of nature or were they purposely sent to Earth to destroy it?
Since their first discovery, several other Ajin appearances have been reported worldwide. For the majority of citizens though, these creatures are less reality and more of something you see every once in a while in a textbook or on the news. This is how it was for the young protagonist of the series, Kei. That was until a sudden turn of events changed his life forever...
Ajin is a supernatural anime full of complex ideas about life and death, as well as the dark side of human nature. What does it mean to truly be human?
It's not entirely clear to me whether the first movie covers the anime series and the other two are new content, or whether the three movies are supposed to be recaps of the whole series (maybe there are supposed to be three anime seasons of Ajin, and each of the movies recaps one of them?).
What's the relation between the manga, the anime series, and the movies (given that two of the three movies haven't been released yet, that part may be harder to answer)? Are the adaptations faithful to the manga?