Alert is a new feature in 3.0 to send an email to users about a specific change I just made.
It is similar but different than watch
- Watch: pull, watching all future changes
- alert: push an alert about a change that is being made now
It is similar but different than tell a friend
- Tell a friend: Email someone about a specific URL (and possibly a comment)
- alert: Email a number of people about a change that is being made now