A WatchKit app is a user launchable app that appears on the Apple Watch home screen. A WatchKit app is the user’s main way of viewing and interacting with your data. It provides the means to view your data and optionally to manipulate or interact with that data. Depending on the data, a WatchKit app might present only a subset of the data presented by its containing iOS app.
A WatchKit app acts as the public face of your app but it works in tandem with your WatchKit extension, which is the brains of the operation. The WatchKit app contains only the storyboards and resource files associated with your app’s user interface. The WatchKit extension contains the code for managing content, responding to user interactions, and updating your user interface. And because the extension runs on the user’s iPhone, it can coordinate with your iOS app as needed to perform more sophisticated tasks.