Apple:
WatchKit apps have two parts: A WatchKit extension that runs on iPhone and a set of user interface resources that are installed on Apple Watch. When your app is launched on Apple Watch, the WatchKit extension on iPhone runs in the background to update the user interface and respond to user interactions. WatchKit provides three opportunities to extend your iPhone app to Apple Watch: WatchKit apps, Glances, and actionable notifications.
Extremely excited for the opportunities WatchKit and the Apple Watch will open up for third-party developers and users.
Also nice that we get more than just simple notifications out of the gate. Although it does seem that communication is only one-way for now:
Write-only pic.twitter.com/1uaLptfn9B
— Steve T-S (@stroughtonsmith) November 18, 2014One fun note is that Apple has also released the system font for Apple Watch, called San Francisco. Gruber points out another memorable typeface with that name.
Make sure you check out the WatchKit Human Interface Guidelines, the WatchKit Programming Guide and the WatchKit Framework Reference.