Flipbook turns your iPhone into an on the go animation studio. It doesn't take much skill or anything to get started with Flipbook, but it will take some when you start to master it. This app allows you to create a series of pages that your iPhone will then animate one after another, making your own little shorts.
There are two versions of this app, a lite version and a $9.99 version. The lite version only limits you on a few things (number of flipbooks, total number of pages, layers per page) but gives you a great grasp on how the app works. There are plenty of different options for you to take advantage of, ranging from how thick your brush is to placing a shadow of the previous image on your current page (potato skinning).
Flipbook introduces a lot of new gestures onto the iphone, including a three finger movement that actually changes the size of your brush or eraser on the fly. The full version even allows you to upload your flipbooks to their website for everyone to see. You can be as simple or as complex with this app as you'd like, it's really up to you.
If you're not great with art based apps, you should probably stick with the lite version until you hit your limitations. The app is definitely worth purchasing if you're interested in pushing your knowledge or applying something you already know. You should definitely check it out, even if you just make a stick man walk across the screen.