The Recent log messages report is probably the most commonly used of the reports available in Drupal core. Any module is able to report its activity here in a standardized way, whether it is for error reporting or simply informational details.
Click on the Recent log messages link, and you'll see a report similar to this:
Due to the fact that any module can insert messages here, the display can be a little confusing. Each log entry has the following fields:
Type: This is defined by the module that recorded the log entry and is most often the module name.
Date: The date and precise time (not shown in the preceding screenshot to save space) of the recorded log entry.
Message: This field displays more detailed information associated with the log entry. This can be a truncated summary, and if you click on the message, you can often get more information.
User: If the message was generated from a logged in session, in which the user was logged in, this will show the user's name...