SaaS, PaaS and IaaS models
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IaaS cloud is the new Hardware. 

In the IaaS model, the provider delivers computer hardware infrastructure as a service. Instead of purchasing servers and other hardware resources and installing in their data center, customers rent those resources. Often people confuse IaaS with hosting services provided by Internet Service Providers (ISPs), as both the services are similar. The key differentiator is the instant scalability of infrastructure on demand in the case of IaaS.

SaaS is the new software.

In the SaaS model, the provider licenses an application to customers for use as a service on demand. Subscribers use and pay for the time application is used either on a pay-per-user or pay-per-month model. The SaaS provider takes care of all the maintenance of the software, the data center, the backup, and the support of the system. For example, in the case of Google Apps, users don’t have to install software on their computers, and they don’t have to install and maintain expensive servers and the associated software they require to run. Instead, they simply access Google’s services in a web browser. Everything is on Google’s infrastructure — the software, the data, the backups, everything — and is therefore accessible in the cloud from anywhere.

PaaS is the new middleware.

In the PaaS model, a computing environment and the related development and deployment stack needed to deliver a solution is provided to the customer. Software developers can build new applications or extend existing applications without requiring the need to purchase development, testing, or production server infrastructure.