There are three main types of cloud computing models.
Infrastructure as a Service (IaaS)
Infrastructure as a Service offers the basic building blocks for cloud IT and generally provides access to networking functions, computers (virtual or physical), and data storage capacity. IaaS provides the highest level of flexibility and control over your IT resources and is most similar to existing IT systems that many development teams and developers are already familiar with.
Platform as a Service (PaaS)
Platforms as a Service eliminate the need for organizations to manage the underlying infrastructure (usually hardware and operating systems) and allow you to focus on deploying and managing your applications. This helps you become more productive as you no longer need to worry about resource procurement, capacity planning, software maintenance, patching, or any other unspecialized tasks associated with running your application.
Software as a Service (SaaS)
Software as a Service provides you with a fully developed product that is administered and operated by the service provider. In most cases, people referring to SaaS are talking about end-user applications. With a SaaS offering you don't need to think about how the service is managed or how the underlying infrastructure is maintained; you simply need to think about how you will use that specific software.