Have you heard people talking about "tier three data centers" etc. and wondered what planet they were from? Well The Uptime Institute has the answer - a short white paper explaining the characteristics of each of the four tiers, handily numbered I (basic) through IV (fault tolerant) for the Romans amongst us.
It's interesting that the top-of-the-range fault tolerant/highly resistant tier IV data center listed in one of the tables achieved 99.995% availability (down for just under half an hour per year!), still short of the "five nines" availability that people with very deep pockets sometimes insist they need.