It started out as a Batman Vs Superman story in the first trailer and we thought “Why should two superheroes be fighting?”
But this trailer shows that the two join forces with Wonder Woman to fight a common foe. The foe is Doomsday, the villain from Superman’s past (an ancient being from Krytpton). It would seem that this supervillain was conjured up by Lex Luthor (played by Jesse Eisenberg)

Hmmm… Are we witnessing the dawn (birth) of the Justice League? For those who don’t know about the Justice League from the comic books of old, the team is an assemblage of superheroes who join together as the Justice League. The seven original members were Aquaman, Batman, the Flash, Green Lantern, the Martian Manhunter, Superman, and Wonder Woman

Anyway Batman v Superman: Dawn of Justice arrives in cinemas March 25, 2016. Watch out.
https://www.youtube.com/watch?v=yViIi3gie2c