Remember PostSecret? That was an app based on the PostSecret website, where people were encouraged to share secrets anonymously, with pictures and words. The app got discontinued due to potential abuses; the founder, Frank Warren, pulled it from the App Store when he thought it had gone too far.
Whisper, then, uses the same idea: allow people to post pictures with meme-like text over the top to anonymously share secret things they can’t get out there on regular social networking sites. What Whisper does differently, though, is to put the private messaging behind a paywall. You’ll need to pony up six bucks a month to be able to message users of the service privately.