Serving up the "social" in Social Coding, the Activity APIs provide access to notifications, subscriptions, and timelines.
List of Atom feeds available for the authenticated user.
Notifications of new comments are delivered to users. The Notifications API lets you view these notifications and mark them as read.
Repository Starring is a feature that lets users bookmark repositories. Stars are shown next to repositories to show an approximate level of interest. Stars have no effect on notifications or the activity feed.
Watching a Repository registers the user to receive notifications on new discussions, as well as events in the user's activity feed.