The app lets you listen to the movie's sound track, see photos, watch trailers and best of all - interact with an on-screen image of the monster Carol. Carol can be tickled, hit, he'll throw things at you, you can put him to sleep at night and he'll eat photos of the contacts on your phone.
It's a fun, free little app and is probably going to be very effective in convincing the kids that try it that the movie is a must-see. Parents familiar with the book probably didn't need any convincing.
Either way, there's something I think all of us can enjoy about listening to Karen O sing in the background while watching a big cuddly monster eat photos of people we know from our phone.