Reply is a D7 answer to entity comments.
Reply is basically a fieldable entity so you can create a 'review', 'comment', 'forum post' or whatever you like by creating bundles and attaching fields to them. Such bundles can be attached to any fieldable entity by 'reply field'.
You can set access for reading and writing, position the form to the same page as entity or to a custom page, allow or deny replying to replies(= no hierarchy), choose threaded or flatted list. Such settings can be set for bundle, field, instance and entity with build-in inheritance which makes if a four level hierarchy settings system.
Most common usage of this module is comment replacement and reviews.