Hibernate logs various events using Apache commons-logging. Hibernate logs are very interesting to read if you want to do some troubleshooting or performance improvement. Hibernate uses commons-logging service for logging. It actually sends the log to either Log4j or to JDK1.4 logging.
If you want hibernate to log using Log4j, then put log4j.jar into class path along with log4j.properties. Since log4j has almost become a standard these days, I would advice using that.