The guestbook/comments block allows users to post comments to your website. It is generally used for blog posts or things where you want feedback from your users. A more advanced version is available in the marketplace for free.

• Title is displayed above the guestbook on the page.
• Date Format uses the standard PHP date string formatting controls to let you define how post dates/time are shown.
• Comments Require Approval - if on people will be allowed to comment but an admin will have to visit the page and hit approve on each comment before it is approved. There is no central approval interface. 
• Posting Comments is Enabled? - sometimes after a guestbook has had an active discussion it makes sense to stop letting people post but keep the existing posts up for historical reasons. 
• Authentication required - Email only lets anyone post if they put anything that looks like an email into the email field. Users must login requires posters to be logged in members to your site.
• Captcha required to post- Captcha is a form of verification to make sure that your posters are not spam bots that are trying to pollute your site with bad content. Concrete5 uses the SecureImage captcha program to do so. There is also preliminary work to integrate more advanced bot detection to keep your site spam free!
• Notify email- The email that you provide here will be notified each time a comment is posted, useful for responding to visitors. If you set no email no email will be sent. If there is an email sent, there will be a log entry added that contains the posters email as well if you wish to contact them through email.