The Comments feature adds online community-building functionality to any Page on your site. Comments allow Visitors and Users to interact with your site.
If Comments are enabled for a Page, then anyone who can view a Page can leave a Comment (and therefore become a Contributor), and read any published Comments from other Contributors. Comments can be automatically published, or the Page's Moderator (a User with edit rights to the Page), can be alerted via e-mail to new Comments with a link to instantly edit, delete or publish the Comment.
Comments can be "watched" too, so anyone can request to receive an email alert whenever a new comment is posted to a Page. This is a great way to create support ticket systems, forums, and any other private/group/public conversation threads.