Drupal has a pretty robust comment system. You can set up basic commenting, threads, anonymous commenting, sorting of comments, and more all with the core Comment Module included in a basic Drupal installation.
Turning Comments on and Off
Drupal has comments turned on by default. You turn the comments on and off by enabling and disabling the comment module.
Comments Menu (in the Content management section)
This is where you set up most of the options available for comments.
The first thing you'll see is all of the published comments within your site. From here you can unpublish any comment, or you can delete them outright.
Next to this tab is the Approval queue. This is where all the comments are that have been submitted, but not yet approved by you. Just check the comments you want to approve and those comments are now published.
The settings tab is where you can set the way the comments are viewed. This is divided into two parts;:Viewing Options, and Posting Settings
The viewing options control how the comments are posted. Some of the options include a threaded list, a flat list, and subject only. You can also sort the comments by newest first, or oldest first.
The comment controls allows the viewer to set these options themselves.
The posting settings control more of what is included in the comments as opposed to what is shown.
This setting allows you to set various options for anonymous users who want to leave comments. You can tell Drupal to force anonymous users that in order to post a comment, they must leave there name and email. This does not give them a username and password to access Drupal as a user. So they are still anonymous to Drupal.