You can sort of guess what each log level those by its name. "debug" is
used for when the program isn't behaving the way it's supposed to, and the
developers of the program want to know why. It might dump the contents of
variables, or notify entry and exit of methods, for example. "info" is just
to inform the user about what the program is doing. "error" is used when
something bad happens, and "emerg[ency]" is for when something REALLY bad
happens. Perhaps with "error", the program can try to recover, but it the
user or administrator should fix something soon. With "emerg", the program
will probably halt then and there.

   - Oliver