The movie stars Fox as the title character, whose body is possessed by a demon post-accidental death. That lets her take revenge on her various tormentors, though of course demon-Jennifer then just starts killing for the fun of it.
Why, yes, Diablo Cody did write the screenplay.
Warning: The video has cursin’, killin’ and brief nudity. Pretty much everything you’d ask from a movie by Diablo Cody starring Megan Fox.