Notification means subscribing to a photo or a category if you want overall notification of any action in a specific category. This has been a feature since day 1.
Users whom post the actual photo can elect whether or not they want notification of comments on there photo.
If you hit the notification link when viewing a photo you will get an email when a comment is made on a specific photo
if you hit the subscribe to category link when viewing a category any new photo or new comment made in that category you will get an email.