In Short

Extensibility in iOS 8 is arguably the biggest thing to happen to the iPhone and iPad since the advent of the App Store. With Extensibility Apple has enabled developers to extend the interface and functionality of their apps into Notification Center, Share Sheets, the input system, the new DocumentPicker, and into other apps. This allows for everything from widgets to custom keyboards and more. Most impressively, Apple has done all this while maintaining the security of iOS. Extensions run in the same sandbox as the apps they extend but within separate containers. That means you get all the functionality you want while maintain the privacy and security of the system you cherish.