Data Center Classification

The TIA-942:Data Center Standards Overview describes the requirements for the data center infrastructure. The simplest is a Tier 1 data center, which is basically a server room, following basic guidelines for the installation of computer systems. The most stringent level is a Tier 4 data center, which is designed to host mission critical computer systems, with fully redundant subsystems and compartmentalized security zones controlled by biometric access controls methods. Another consideration is the placement of the data center in a subterranean context, for data security as well as environmental considerations such as cooling requirements.[2]
The four levels are defined, and copyrighted, by the Uptime Institute, a Santa Fe, New Mexico-based think tank and professional services organization. The levels describe the availability of data from the hardware at a location. The higher the tier, the greater the accessibility. The levels are:

Tier Level Requirements
1
• Single non-redundant distribution path serving the IT equipments
• Non-redundant capacity components
• Basic site infrastructure guaranteeing 99.671% availability
2
• Fulfils all Tier 1 requirements
• Redundant site infrastructure capacity components guaranteeing 99.741% availability
3
• Fulfils all Tier 1 & Tier 2 requirements
• Multiple independent distribution paths serving the IT equipments
• All IT equipments must be dual-powered and fully compatible with the topology of a site's architecture
• Concurrently maintainable site infrastructure guaranteeing 99.982% availability
4
• Fulfils all Tier 1, Tier 2 and Tier 3 requirements
• All cooling equipment is independently dual-powered, including chillers and Heating, Ventilating and Air Conditioning (HVAC) systems
• Fault tolerant site infrastructure with electrical power storage and distribution facilities guaranteeing 99.995% availability

source : http://en.wikipedia.org/wiki/Data_center