The practice of an organization making software functionality available to the public through a web service became known as Software as a Service (SaaS). With SaaS, companies are offering many types of services. Hardware that can be accessed over the Internet, termed Infrastructure as a Service (IaaS) and extensible platforms, called Platform as a Service (PaaS). Most cloud services fit into the three categories of SaaS, PaaS, or IaaS (Mell & Grance, 2011) and are depicted in Figure 6. Services that fall outside of the general three categories of cloud services are often called Everything as a Service (EaaS) or as a Service (aaS) (Banerjee et al., 2011).
Figure 6. Cloud Service Types
There is no official definition of cloud computing. The National Institute of Standards and Technology (NIST) defines cloud computing as having five distinct characteristics (Mell & Grance, 2011). First, cloud computing is on-demand and is self-service. Consumers of cloud services can sign up online and immediately start using the cloud service. A cloud service does not require salespeople to negotiate the price, legal people to work out the contract, or implementation specialists to help the customer work with the system. Customers signing up for cloud services have a set price, contract, and implementation that they will use. Cloud computing must be accessible over broadband communication channels. Cloud computing’s resources are pooled and computational capacity can expand rapidly. Many cloud computing providers use grid computing to expand rapidly. Finally, cloud computing is a measured service. These measures help cloud computing services to rapidly expand to meet the needs of consumers or throttle consumers who are using enough services to slow the system down for other customers.
Figure 7. Cloud Computing Principles