A log book is a writing tableau of some kind located in a geocache by which a visitor to said geocache is able to record their visit.
A log book serves as physical evidence that a geocacher has indeed found the cache. Some cache owners require as a minimum that the log book be signed by a visitor in order to allow the visitor to count the cache as a legitimate find.
In general, most geocaches contain a log book, even if that is the only thing in the cache. As a rule of etiquette, a pen or pencil is included along with the log book, but it is not required. This may be the case with a microcache.
Often a log book will be separated from the rest of the cache items by some kind of sack designed to keep it dry. A common sack would be a self-sealing plastic bag. This allows the visitor to quickly locate the log book within the cache.
Log books for larger geocaches are often notebooks, such as spiral-bound notebooks or blank journal books. This allows the visitor space to include any comments on the cache, such as how the hunt was conducted and how the visitor felt upon finding the cache, in addition to recording their name and the date of visit.
Log book styles vary based on the preferences of a cache owner. They can be elaborate and cost a significant amount of money, or they can be rather simple and have minimal cost. Some vendors sell special geocache log books, which feature details like weather-proof paper or geocaching-related information printed on the covers.
If you cannot find a spiral-bound notebook that is the right size for your cache, or if you need an unusual size/shape notebook for a particular container, then you can cut a larger notebook to size with a bandsaw. Just be sure to cut the spiral wire cleanly with wire cutters, and to turn under the cut ends.
Log sheets are often used in smaller caches. The log is a simple piece of paper, usually with ruled spaces for visitors to record their names and the dates of their visits. Log sheets for micro-caches (and especially for nano-caches) may be designed specifically to fit into the container, and may be printed as part of the stash note.
Log books and online logging
Most geocaching websites include a section by which a visitor is allowed to record their visit on the cache's description page. This is usually consider separate from the physical log, and visitors are encouraged to submit logs in both places. A usual difference is that an online log tends to refrain from providing blatent information regarding the location of a geocache, while the physical log book may be written in by the visitor without this restriction, usually with the notion that anyone that has found the cache no longer has any need to avoid such spoilers!