Infrastructure as a Service (IaaS)
At the basis of Cloud Computing we find IaaS service. With that model the customer has the possibility to access directly to the infrastructure and perform some activities. The Cloud provider makes available to clients the virtualized machine (or machines, if the system is more complex) and enables to take advantage of the pay-per-use model, according to which you only pay the resources you have actually used. So, at this level, the customer has the direct access and control of the Cloud infrastructure.
Platform as a Service (PaaS)
The PaaS model is characterized by the provison of a Cloud platform on which the customer can develop and deploy his applications. In that case the user does not have the control of the Cloud infrastructure but he can configure and manage the applications on the platform provided in total freedom, without worrying about the underlying system. For these reasons, PaaS service represents the most popular solution for web agencies and developers.
Software as a Service (SaaS)
SaaS is a model of software distribution via web. Also here the client has no access or control on the Cloud infrastructure. It is turn-key solution to use software programs easily and avoid all the troubles deriving by the IT system management.