A Logbook was originally a book for recording readings from the log, and is used to determine the distance a ship traveled within a certain amount of time. The readings of the log have been recorded in equal times to give the distance traveled with respect to a given start position.
Today's ship's log has grown to contain other types of information, and is a record of data relating to a ship or submarine, such as weather conditions, crew complement or what ports were docked at and when. It is essential to traditional navigation, and must be filled in at least daily.
Most Admiralties specify that logs are kept to provide a record of events, and to help crews navigate should radio, radar or the GPS fail. Examination of a log is often used to try to explain some sort of disaster, in much the same way as a "black box" is used on airplanes (see Mary Celeste).
The term logbook has spread to a wide variety of other endeavors and logbooks are widely used for e.g. complex machines like nuclear plants or particle accelerators where one is more and more using a computer based version of a logbook called electronic logbook (see Electronic logbook). In military terms, a logbook is a series of official and legally binding documents. Each document (usually arranged by date) is marked with the time of an event or action of significance.
For Amateur radio the logbook is where the hams register their QSO and radio activity. There are several programs to help radio operators in the management of their logbook.