The OntoBroker folder contains a file "log4j.properties". The first line:
log4j.rootCategory=INFO, console, file
says that only INFO messages are logged and that these INFO messages should be logged to both the console and the file. The last line:
in the log4j.properties file specifies the name of the file where the log messages are logged to.
The layout.* sections specify the style of the output. E.g. [%t] says that the name of the thread which is logging the message should be put within square brackets at the beginning of the log message.
The log4j logging library has sophisticated logging features (e.g. logging to the Windows XP system log etc.). For more information, see the official manual page: