Log files contain diagnostic log messages and can be found in the Log folder of the Composer. There are many different kinds of logs that refer to different parts of the composer and activities.
The Composer log can be found in the Application folder of the log folder. It contains information about the behaviour of the application.
The Activity log can be found in the Activities folder of the log folder. It contains information about the behaviour of the workflow execution and activities. To find further information on errors that appear during some activities, this also is the place to go.
To prevent certain activities from being written to the log, the option 'Disable Log' can be checked.