I can point you to a brief explanation in the Yum User Guide: http://docs.fedoraproject.org/yum/en...m-caching.html
The command clean dbcache deletes all the sqlite databases (you got an error message about one of these files). Look at the content of the yum cache on your system: it should be under /var/cache/yum. For each repository it stores the installed packages and all the related information. If you clean the cache, yum will rebuild it at the next run.
The advantage of the yum cache is to perform faster operations. The dis-advantage is that the cache requires some extra disk space in the /var partition.