A guest book is traditionally a type of ledger on which a record of guests to a particular establishment (common examples being bed and breakfasts and museums) is voluntarily updated by the guests themselves upon leaving. It would generally consist of the guest's name, the date, and any (polite) comments the guest had about the place:

John Doe | November 10, 2004 | I love this place!

More recently, this term is used on websites for an electronic equivalent of the object described above. The visitor (again, almost always vonluntarily) will post a comment with his/her (user)name. The rules of ettiquette are generally held to be the same.