The SMART test has reported a failure, what does that mean?

The implementation of SMART (Self-Monitoring, Analysis and Reporting Technology) varies from manufacturer to manufacturer.  Attributes 1 to 13 are fairly standardized, and any failures there probably warrant replacing the drive.  Other attributes may not be cause for concern.  For example, SMART attribute 190 is implemented differently on Western Digital drives than on others.  See this Wikipedia article for more information.