The 'site down' alert is sent when either the whole or part of a web site is unavailable. It is sent when the balancer thinks that there are no functioning back-end machines for your site. If you have set up mapping rules, e.g. if URLs matching '/images/.*' are handled by just one machine, and that machine fails, then the balancer will also send the 'site down' message. The alert means that at least parts of your site are not usable because of the failures.
The alert 'back-end failed' is sent when a back-end machine is found to be non-functional, but the failure of this one machine is not causing any downtime for any part of your site. In either case, you are advised to check out the problem and fix it to maintain the health of your site.
