PS Express is an easy to use, very basic, free photo editor made by Adobe. You can choose from a variety of one-touch effects, adjust exposure, brightness, contrast and more. Use your finger to drag across the screen to crop, rotate, or adjust color. There is multiple undos and a copy of your original file is always saved. There are a few artistic filters and effects for creativity as well as some borders. Unfortunately most of them will need to be purchased in app or at the store for a fee. But the basic editing tools are there in a very simple interface. If you are looking for a more advanced Photoshop app check out PS Touch.
Get it at the App store: https://itunes.apple.com/us/app/adobe-photoshop-express/id331975235?mt=8
Check it out at Adobe: http://www.photoshop.com/products/mobile/express