Stack Exchange provides notifications about some things, like comments on your posts and answers to your questions. If you have a question about how these relate to the API, or how the API relates to them, use this tag.
You can also use this tag when directly dealing with notifications as represented by the API. The following API methods return notification objects:
/notifications(network-wide)/notifications/unread(network-wide)/users/{id}/notificationsand/me/notifications(per-site)/users/{id}/notifications/unreadand/me/notifications/unread(per-site)
All of these methods require authorisation - that is, an access token with read_inbox scope. (This means that the /users/{id} forms of these methods are pretty much useless, as the only usable ID is your own, and the /me forms can do that without needing you to feed your ID in again.)