From Koha Wiki
Koha will have a logging module (Koha::Utils::Logger). Please have a look at Bug 8190 for more information.
How to choose your log level?
8 levels are available: debug, info, normal, warning, error, critical, alert, unusable.
Debugging information used by developpers.
Example: Display a variable's value.
An information message.
Example: The user's operations, indexation of data, etc.
A normal operation is reported.
It is something quite important.
The application could have strange behaviours.
Example: A regexp fails, a syspref does not exist, data is corrupted, etc.
An user error occurred. Koha catch the error and does not crash but something is wrong.
Example: Bad field format, SQL error, bad right on a file, etc.
Koha is broken. The interface still responds but the service is degradated.
Example: a field is missing in the DB but the application is usable.
Koha is going to be unusable, an action must be taken immediately.
The application is unusable.
Example: an important module is not installed, an important field is missing in the DB, the connection to the DB fails, etc.