Public data centers:

A public data center (also called internet data centers) provide services ranging from equipment colocation to managed web-hosting. Clients typically access their data and applications via the internet.
Typically, data centers can be classified in tiers, with tier 1 being the most basic and inexpensive, and tier 4 being the most robust and costly. The more ‘mission critical’ an application is, the more redundancy, robustness and security are required for the data center. A tier 1 data center does not necessarily need to have redundant power and cooling infrastructures. It only needs a lock for security and can tolerate upto 28.8 hours of downtime per year.