A guestbook is an attachment to a website which allows visitors to write comments, suggestions or messages which can be seen by the site's creator and, on most cases, by other visitors.
Guestbooks are very useful for small or medium-sized sites (for a large site, a web board could be a better choice) for they provide a simple way in which users can communicate with the site's creator, and sometimes encourage them to return more often. They can also be a source of contacts.