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. Most 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, a geocache has the minimum requirement of containing a log book, even if that is the only thing in the cache. As a rule of ettiquette, a writing device of some kind, such as 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 render included protection from weather. 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 tend to vary in terms of size, shape, and form. For a standard-sized geocache (one that has other items included), a common style of log book is a notebook, such as a spiral-bound notebook, or sometimes a blank journal book. 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.
For smaller caches (usually microcaches), the log book may consisted of a small piece of paper upon which there is room only for a visitor to record their name and date visited. Often these log books are partitioned in order to limit the visitor's entry in order to such.
Log book styles are varying 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 geocaching vendors sell special log books that are designed for placement in geocaches, having qualities such as additional weather-proofing and lining specific to geocaching.
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!