What are rpmnew & rpmsave files?

Issue

• What are rpmnew & rpmsave files?
• In which case (if not all) should one systematically analyse the changes brought by a new package when a rpmsave file is created? (based on the severity of the errata? Based on the type of errata? Based on the type of package? etc...)
• Can one simply ignore/delete rpmnew files?
• Other than by looking at the output of the yum update command, is there a command or method that would be best to identify packages that generated such files locally on the patched server?
• What are the best practices commonly seen/used in large Enterprises for dealing with rpmsave and rpmnew files?