Much like how the Marvel movies all work on their own but build a bigger ongoing story, Justice League is part of the DC Cinematic Universe, and continues the story begun with Man Of Steel, Batman V Superman: Dawn Of Justice, Suicide Squad and Wonder Woman.
The Caped Crusader (played by Ben Affleck) and Diana of Themyscira (played by Gal Gadot) are back, along with some of the pals they had in their own movies
The Flash is currently in his own TV show, but for some reason (probably a needlessly complicated one involving really boring contracts and stuff), the movie Flash is totally different. The TV one is played by Grant Gustin, the movie one by Ezra Miller, and they're just completely separate.
After being briefly seen in Dawn Of Justice, it's time for these two to kick loads of (wet/robotic) butt! Aquaman (played by Jason Momoa) is king of the undersea world of Atlantic, while Cyborg (Ray Fisher) is a super-intelligent half-robot!
Superman, er, didn't have the best time at the end of Batman Vs Superman: Dawn Of Justice. He, uh, look, if you've not seen it stop reading this bit and jump to the next one. But he died in it. However, in the comics, Superman is kind of the centre of the Justice League, and the actor who plays him, Henry Cavill, has definitely been seen on set, so some sort of resurrection might be on the cards!
Believe it or not, Justice League was initially meant to come out in 2007! It got as far as getting a whole cast and crew together, but was cancelled at the last minute and the whole idea was re-thought, which eventually led to Man Of Steel and the beginning of this whole series of movies!
It's not out of the question that The Rock might make a cameo as the villain Black Adam, ahead of a movie about the character that's happening in a few years time. Like, The Rock seems to enjoy being in stuff, and everything's better if The Rock is in it, right?
There have been rumours that Green Lantern, Darkseid and Deathstroke might appear – guess we'll only find out in the cinema!
Originally, rather than Cyborg, the film was set to include the green-skinned Martian Manhunter (aka J'onn J'onnz), but it was decided that this was a bit too silly. Sillier than a guy dressed as a bat? Sillier than an indestructible lady who's half god? Sillier than a dude who lives at the bottom of the ocean and looks like a wrestler? Dunno...
The baddie in Justice League is Steppenwolf, an immortal extraterrestrial warlord who likes nothing better than walloping people with an axe. What a rotter!
The makers of the movie are so confident about it that there's a sequel in the works, as well as solo movies featuring Cyborg and Aquaman, sequels to Wonder Woman and Suicide Squad, and a spin-off called Justice League Dark, about a gang of deathy, grim, kind of monstrous superheroes!
It's coming out for both normal cinemas and really massive IMAX screens, and look how nice this poster is!