There are two types of iOS photo editors: the type that lets you fiddle and adjust images in very precise increments, and the other type that provides a way to enhance your photos with a minimum of clicks.
Prettify (free with some in-app options) belongs to the latter category, and it does its job with flair and ease. Open a photo, click enhance and you get Hi-Def, Illuminate and Color Fix options. Each enhances the image in a different way: Hi-Def appears to adjust curves and saturation; Illuminate adjusts brightness and contrast; and Color Fix rebalances the color. Each fix can be used separately or in an additive way with another fix.
Beyond those fixes there are many Instagram-like filters. There's also a tool to defocus or sharpen selectively, and there are the standard cropping and saturation tools. Portraits can be retouched with a teeth-whitening filter, red-eye removal and a blemish eliminator.