Infrastructure as a Service, or IaaS, gives business access to vital web architecture, such as storage space, servers, and connections, without the business need of purchasing and managing this internet infrastructure themselves. Because of the economies of scale and specialization involved, this can be to the benefit of both the business providing the infrastructure and the one using it. In particular, IaaS allows an internet business a way to develop and grow on demand. Both PaaS and SaaS clouds are grounded in IaaS clouds, as the company providing the software as service is also providing the infrastructure to run the software. Choosing to use an IaaS cloud demands a willingness to put up with complexity, but with that complexity comes flexibility. Amazon EC2 and Rackspace Cloud are examples of IaaS.
Platform as a Service (PaaS) clouds are created, many times inside IaaS Clouds by specialists to render the scalability and deployment of any application trivial and to help make your expenses scalable and predictable. Some examples of a PaaS system include: Mosso, Google App Engine, and Force.com. The chief benefit of a service like this is that for as little as no money you can initiate your application with no stress more than basic development and maybe a little porting if you are dealing with an existing app. Furthermore, PaaS allows a lot of scalability by design because it is based on cloud computing as defined earlier in the article. If you want a lean operations staff, a PaaS can be very useful if your app will capitulate. The most important negative of using a PaaS Cloud provider is that these services may implement some restrictions or trade-offs that will not work with your product under any circumstances.
Software as a Service (SaaS) is relatively mature, and the phrase’s use predates that of cloud computing. Cloud applications allow the cloud to be leveraged for software architecture, reducing the burdens of maintenance, support, and operations by having the application run on computers belonging to the vendor. GMail and Salesforce are among examples of SaaS run as clouds, but not all SaaS has to be based in cloud computing.
See more about cloud computing/SaaS definitions at our Cleaning Up a SaaS Terminology Mess