Basically, any app that serves notifications on your Android smartphone or tablet can be extended to show notifications on a wearable device running Android Wear with minimal effort. Jordan mentioned by adding a few lines of code, devs can "deliver even richer experiences on the wearable by using stacks, pages, and voice replies."
The notifications are identified by the app icon, which is featured in the top right corner. The notifications themselves seem to take up the lower half of the screen, with Google also demoing a right-to-left swipe gesture that allows you to view longer notifications over multiple screens. If you're interested in knowing how these notifications can be enabled, check out the video below: