Critical conditions. (i.e. application component unavailable) Will page a human to address the issue.
Only interesting to developers. Causes no disruption and represents no deviation.
Occurrences that do not require immediate action but should typically be logged and monitored. Will flag for human review but not immediate action.
Informational messages that highlight the progress of the application at coarse-grained level.
Normal but significant events. (i.e. user accesses a privileged location)
Exceptional occurrences that are not errors. (i.e. user attempts to access a privileged location but is denied)
The severity of the event that you're logging