So, before I got an Iphone myself, I really couldn't have told you what an app was or what they might do-- but gradually I've acquired more & the latest ones I'm playing with (totally trial & error, & I'm sure I'm at the tip of the iceberg of their potential) are 'Instagram' & PhotoStudio. Both allow you to edit & enhance your Iphone's photos with filters & effects.
Above is the same photo (rose: 'Singing in the Rain') processed with 2 different techniques. Fun!
Below, the original photo, so you can see that the auto-flash was already doing something kinda crazy to those dewdrops.