The higher the level (with debug being the highest), the more amount of information is recorded in the log.  We don't recommend leaving it set to a high level generally, just because of the amount of extra information it records, particularly potentially personal information if it is recording what you type and what it reads.  Note that no matter what level the log is set to, nothing is ever done with it - ie it is never sent anywhere unless you attach it to an email etc (which I might ask you to do for a bug I can't replicate).

Kind regards

Quentin.