Tiers, or levels, are ways to differentiate the requirements of each type of data center operator, with a focus on redundant components, critical load distribution paths, cooling, and many other specifications. As it stands now, there are four tiers, and as you would expect, they are defined precisely.

Tier 1 to 4 data center is nothing but a standardized methodology used to define uptime of data center.

Tier 4 data center considered as most robust and less prone to failures. Tier 4 is designed to host mission critical servers and computer systems, with fully redundant subsystems (cooling, power, network links, storage etc) and compartmentalized security zones controlled by biometric access controls methods. Naturally, the simplest is a Tier 1 data center used by small business or shops.

Tier 1 = Non-redundant capacity components (single uplink and servers).
Tier 2 = Tier 1 + Redundant capacity components.
Tier 3 = Tier 1 + Tier 2 + Dual-powered equipments and multiple uplinks.
Tier 4 = Tier 1 + Tier 2 + Tier 3 + all components are fully fault-tolerant including uplinks, storage, chillers, HVAC systems, servers etc. Everything is dual-powered.

Data Center Availability According To Tiers

The levels also describes the availability of data from the hardware at a location as follows:

Tier 1: Guaranteeing 99.671% availability.
Tier 2: Guaranteeing 99.741% availability.
Tier 3: Guaranteeing 99.982% availability.
Tier 4: Guaranteeing 99.995% availability.