Construct and manage your app’s user interface for watchOS.
Respond to events generated by interface objects, gestures, and the system.
The WatchKit app
The WatchKit app is an executable app to be installed on your watch, and you can install or uninstall it from your iPhone. The WatchKit app contains the storyboard file and resources files. It doesn't contain any source code, just the interface and resource files.
The WatchKit extension:
This extension runs on the iPhone and has the InterfaceControllers file for your storyboard. This extension just contains the model and controller classes. The actions and outlets from the previous WatchKit app will be linked to these controller files in the WatchKit extension.