Data Center Tiers

Tier 1 = Non-redundant capacity components. things such as power have single feeds.
99.671% Uptime
no redundancy
28.8 Hours of downtime per year.

Tier 2 = Tier 1 + Redundant capacity components.
99.749% Uptime
Partial redundancy in power and cooling
Experience 22 hours of downtime per year

Tier 3 = Dual-powered equipment and feeds for power, cooling, and essential services.
99.982% uptime (Tier 3 uptime)
No more than 1.6 hours of downtime per year
N+1 fault tolerant providing at least 72-hour power outage protection

Tier 4 = All components are fully fault-tolerant including uplinks, storage, chillers, HVAC systems, servers etc. Everything is dual-powered.
9.995% uptime per year (Tier 4 uptime)
2N+1 fully redundant infrastructure (the main difference between tier 3 and tier 4 data centers)
96-hour power outage protection
26.3 minutes of annual downtime.

If you want to read up on the N redundancies. https://en.wikipedia.org/wiki/N%2B1_redundancy