What were you doing exactly a year ago? Tweeting a picture of your food? Complaining about the weather? With Timehop, you can look back on past-you with a kind of self-awareness that, quite frankly, might be a little depressing.
This app requires a Facebook login, but can also connect to Twitter, Instagram, and Foursquare. The idea is that Timehop acts as a hub for all the social content you posted exactly one, two, three, and four years ago. It builds a timeline of past-you, which you can share with your networks or just revel in all alone. The app will notify you when a new 'Timehop' is available for viewing in the app.
As for the app itself it seems responsive, but doesn't have much in the way of settings. You can't really complain too much – it gets the job done and is pretty easy on the eyes.