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Software as a Service (SaaS) most closely aligns with what used to be called a managed software service. For example, in the early days of the cloud, hosting companies would offer a hosted version of a certain popular, brand-name enterprise email system. Instead of having to buy, configure, and maintain this email system on your own servers, the hosting company would do it for you on its own servers. All you had to do was configure your client machines to connect to the appropriate server to send and receive email. NIST formalizes this concept in the following description of Software as a Service:

The capability provided to the consumer is to use the provider's applications running on a cloud infrastructure. The applications are accessible from various client devices through a thin client interface such as a web browser (e.g., web-based e-mail), or a program interface. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings.

The Software as a Service model is where the customer of the service accesses the application software that is owned and controlled by the cloud company, which has complete responsibility for the management and support of the application, as shown in Figure 1.7. You, on the other hand, have limited control over the operation and configuration of the software itself. Sticking to the earlier example, you can create and delete email inboxes and control how inbound and outbound emails are processed. But you can't upgrade the software to a newer version.

FIGURE 1.7 SaaS

Business applications are good examples of SaaS and can include customer relationship management, enterprise resource planning, human resources, payroll, and software development applications. Hosted applications such as email or calendars that are accessible from a browser or email client are examples of SaaS.