App Extension - Details

There are predefined extension points and templates for these points applications can utilize

Extension Point Example Today widget Get a quick update or perform a quick task in the today view of notification center Share Post to a sharing website or share content with others Action Manipulate or view content within the context of another app Photo Editing Edit a photo or video within the photos app Storage Provider Choose a document from among the set of documents iOS app can access Custom Keyboard Replace the iOS system keyboard with a custom keyboard for use in all apps.

Applications can share code between extension and containing app using embedded framework.

If Extension wish to share data between containing app, then need to create an app group, and register this app group in developer portal and use the app group in containing app

An App extension lets an app to extend custom functionality and content beyond the current app and make it available to users while they are using other apps. 

A high level extension – host app – containing app integration block diagram is given below

Each template includes extension point specific implementation files and settings.

References:
https://developer.apple.com/library/prerelease/ios/documentation/General/Conceptual/ExtensibilityPG/index.html#//apple_ref/doc/uid/TP40014214-CH20-SW1