You will frequently see a page titled "Testimonials" on websites where the website owners choose to show comments that users have made, usually by emailing them to the company. On those pages, the comments are usually positive because the website owners can choose which emails to use and they can edit them. In a "Visitors Book", the comments are written for anyone to see so if the owner leaves the book on show, anyone can read the comments, both positive and negative.
Many websites have a "Guestbook" where people can leave their name, location and comments. The home page of a website might say "If you wish to leave a comment, please go to the Comments/Guestbook page". There is no one single word or phrase used. My business website only has a "Testimonials" page because we only accept comments by email and I then choose which to post publicly.