User interface with only iPhone in mind in the first place (reason of i UI). User interfaces to be created are iPhone-like yet compatible with most smartphones and tablets and has support for menus, forms, lists, image galleries and more.
Also, there are built-in extensions for caching, getting system information, HTML5 videos or theme switching.
The core code of iUI is lightweight, but a lots of plugins, extensions & themes. Offline cache, Google analytics, system informations, events logs, theme switcher, main mobile OS themes.
iUI is well-documented and has tutorials to get you started easily.