This was announced at CinemaCon, which also featured a new trailer (not yet available online) that showed off classic Spider-Man villains like the Green Goblin and the Kingpin.
According to cbr.com, aside from being animated, this film will have some major differences from other theatrical Spider-Man productions: namely, it will be based on the Ultimate Marvel line of comics, and Johnson’s character will not be the protagonist. Instead, it will focus on the second Ultimate Spider-Man, Miles Morales, to be played by Shameik Moore (of Dope and The Get Down).
The Ultimate Marvel line is set in a different continuity from Marvel’s “normal” comic books, allowing the company to modernize and change various stories. In this version, Peter Parker was Spider-Man until issue #160, when he died in a fight with his nemesis, the Green Goblin. We were then introduced to a new character, Miles, who got spider powers in a similar incident as Peter’s. While reluctant to use these abilities, word of Peter’s death motivated him to take up his mantle as the new Spider-Man.
In the first trailer, we saw a shot of Peter Parker’s grave, and then ended the video with Miles talking to a different Peter, saying “Wait… so how many of us are there?” Combined with the title, this implies some dimension-hopping, and possibly a story line based on the crossover comic Spider-Men (when Miles and the “normal” Peter first met).
Given that Miles is not well-known by non-comic fans, giving him a story alongside Peter makes sense. Assuming it proves successful, Miles will hopefully be able to carry a movie himself in the future.
Sony owns the movie rights to Spider-Man, but with two series about Peter Parker under their belt (a trilogy by Sam Raimi and a duology by Marc Webb), they have now opted to rent his use out to the Marvel Cinematic Universe while instead making spin-offs about lesser-known characters. Aside from Spider-Man: Into the Spider-Verse, they have a Venom movie coming out in October and Silver and Black, a team-up between anti-villains Silver Sable and Black Cat, planned.
Spider-Man: Into the Spider-verse comes out December 14, 2018.