The crux of Impostors is simple: You don't play as Batman or Joker. Instead you play as a highly customizable "imposter" on Team Bats or Team Jokers. "Posing as amateur vigilantes or villains," explains the press release, "gamers will create their very own Bats and Jokerz characters using unprecedented customization options including insane costumes, homebrewed gadgets and a wide range of traditional and imagined weapons." From there, you'll take your character into a series of gametypes and maps, which are "inspired by DC Comics' Batman license."
Impostors is being shown running 4v4 multiplayer at today's preview event, but WBIE tells us that the maximum number of players isn't yet set in stone.
Update: Clarified details based on press release. Additionally, GothamCityImpostors.com is now live.