Though it has been rumored for years, the logistics and continuity of a Justice League movie has prevented it from reaching the active development stage. It came close to happening in 2008 but also hit the writers strike and eventually was shelved. Now though WB president Jeff Robinov has gone on record that a script is in the works and the project is high priority for the studio. There are also plans to spin-off Flash and Wonder Woman into their own films following Justice League.
Justice League is the DC Comics superhero team that includes most prominently Batman, Superman, Green Lantern, Flash, and Wonder Woman. They united in the comics in 1960 and ran until 1987, and have also been featured in various animated series. The idea, at least back in 2008, was to cast younger (and lesser known) actors in the roles which would differentiate them from any ongoing franchises. Now a character such as Batman will have to be completely rebooted following The Dark Knight Rises, and whether they would use Henry Cavill as Superman is in question.