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The Platform as a Service (PaaS) model sits somewhere in between the IaaS and SaaS models, and conceptually it is probably the most difficult to grasp because it doesn't have a clear analog in the data center. Essentially, PaaS gives you a preconfigured computing environment on which to install and run the software of your choice. You have little to no control over the configuration of the OS and VMs on which your application runs. NIST describes it thusly:

The capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages and tools supported by the provider. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, or storage, but has control over the deployed applications and possibly application hosting environment configurations.

A strange but accurate way of saying it is that PaaS offers an operating system as a service on which customers can install their applications, as shown in Figure 1.9. The cloud provider takes responsibility up to and including the operating system, including all hardware and virtualized resources.

FIGURE 1.9 PaaS

Not surprisingly, PaaS is a popular model with software developers because they can deploy their applications quickly without having to mess with provisioning VMs and keeping up with OS maintenance.