The app works by connecting to the Rockstar Social Club. It will allow you to customize your cars from paint jobs, window tints, wheels, hoods, spoilers, engine upgrades, brakes, and so on and so forth. Simply customize it however you want, place your order in iFruit and your customized car will be waiting for you to pick up later on in the game proper.
Another aspect of the iFruit app is taking care of Chop, Franklin's dog buddy in Grand Theft Auto V. in the Chop the Dog game on iFruit, you can feed him, play fetch or tug-of war, teach him tricks, and more. The better you take care of him the better a sidekick he'll be in Grand Theft Auto V. He will be more responsive and find hidden items more quickly if trained well. He will also perform various tricks based off of what he learned in the iFruit app, and you can even trade in credits earned through the app to buy him new collars.
You can download Grand Theft Auto: iFruint for free from the iTunes store. It will soon be coming to Android, Windows Phone, PlayStation Mobile, and Vita devices. A virtual manual will also be made available later today.