Platform as a Service (PaaS) is an environment for running custom applications that provides operating-system level services for accessing the hosting and hardware resources needed in a cloud. Some people have called this category "cloudware."
PaaS should provide applications with dynamic scaling of resources and ready access to an application development architecture (ie., SaaS). Some PaaS have their own dedicated SaaS architecture (Salesforce's Force.com, for example). Most platforms, however, allow the consumer to choose their own SaaS tools.
PaaS is often offered with hosting and other infrastructure services, and in that case, the solution prvider is selling IaaS. However, some PaaS vendors will sell their software for use on a private cloud, in which case, a corporation can add their own infrastructure services.
Among the questions to consider when choosing a PaaS: