The second-lowest log level: less severe than INFO, more severe than TRACE. This is used for debug output, that you may not always have enabled, but that you may want to enable for certain packages where you need more information for debugging.
The median log level: less severe than ERROR and WARN, more severe than DEBUG and TRACE. The standard log level to use for informational output, that most consumers of your logs will be interested in, but that doesn't signal an error in your system.
The lowest log level, for tracing every little detail of an application. This log level will typically be disabled by default, and so will not produce any log output unless explicitly enabled for a logger.
The second-highest log level: less severe than ERROR, more severe than INFO. Use this when a fault has occurred in the system, but it doesn't necessarily require the immediate attention that an ERROR would.