Spartane's explanation is a very good one and I hope you read it. I just want to caution you that just because you get a high quality score with low PIEs and PIFs, it doesn't mean that your disc is error free. Sometimes bad sectors and errors on a disc will not show until you do a transfer rate test or surface scan in Nero CD-DVD Speed. Just take the error scans lightly, or at least never think that a high quality score means your disc will play back without errors/problems in a home DVD player. Error scanning is a very limited and subjective testing method.