Log files are generated by all data processing equipment every time an activity takes place.  It is an electronic fingerprint with an added element: we know at what time that fingerprint was generated, so we are able to reconstruct what happened and in what order. Analyzing logs is the primary way of doing forensics, and properly managed logs can also be used as evidence in a court of law for prosecution purposes.

When you enable logs you can typically specify: 1) the severity level, which essentially specifies how severe the event needs to be to deserve creating a log message and 2) the level of detail captured in the log message, the so-called verbose level.

There are eight standard severity levels, from high-severity level 0 (called emergency, in which only emergency and extremely critical events are logged) to low-severity level 7 (called debug, in which almost any minute event is logged).

Verbose levels are less standards and vary on the vendors, makes and models of equipments.