is a logging system that allows visitors of a website to leave a public comment. Traditionally, the term applied to the actual ledgers held, for that same purpose, at B&Bs and museums.

It is possible in some guestbooks for visitors to express their thoughts about the site or its subject. Generally, they do not require the poster to create a user account, as it is an informal method of dropping off a quick message. The purpose of a website guestbook is to display the kind of visitors the site gets, including the part of the world they reside in, and gain feedback from them. This allows the webmaster to assess and improve their site.

A guestbook is generally a script, which is usually remotely-hosted and written in a language such as Perl, PHP, Python or ASP. Many free guestbook hosts and scripts exist.

Often, e-mail addresses, the visitor's site's URLs and IP addresses are collected, and sometimes published. A good example of this is the free flat file PHP guestbook called BBguestbook which features emoticons or smileys and anti-spam scripts. A guestbook is not intended to be a place for discussion. Due to this, a guestbook is different from a chat room (which is more or less realtime communication), or an Internet forum (which is intended to be a location for discussions), or a blog (which is intended for regular updates and more involved exchanges).

Some newer guestbooks include a map that visually displays the visitors' geographic locations. Such guestbooks are sometimes referred to as guestmaps.