Users can be tagged in comments by typing @ followed by their name (ex: @john). The tagged user will receive both an on-screen notification and an email. Both the notification and email can be customized if necessary from within the settings menu.
From Settings / Users Management / User Tagging, you can configure notification and email settings.
On-screen notifications
In the User Tag Notification window, you can modify the message received on-screen. The system allows you to insert variables to customize the message shown. Variable options are:
%title - for the content's title
%nid - for the content's number
%tagging_username - for the full name of the user that tagged