Infrastructure as a Service (IAAS)
In another approach, you can rent a service offered by the vendor and then configure the service by using the interface provided by the vendor, without having to know what infrastructure the vendor uses to provide that service. This approach is called Software as a Service (SaaS) because you pay to use defined services. For example, Microsoft Exchange Online carries a per-mailbox charge. To configure it, you use a web application supplied by the vendor to request mailboxes, and name and dimension them. You receive a password for that user and nothing else is necessary—users can access their mailboxes immediately.
The third approach is Platform as a Service, or PaaS. In this approach, you rent a platform on which you deploy your applications without configuring the infrastructure and without the limitations of the SaaS approach.
Vendors provide the infrastructure to build solutions, and you rent the hardware such as servers, load balancers, a firewall, and cables. You then configure these remotely and install your solutions on them. You can scale up by requesting more servers and reconfiguring the load balancer without purchasing more hardware. You can scale down at any time by reconfiguring the infrastructure you rented from the cloud service provider. This vendor approach is called Infrastructure as a Service (IaaS) because a customer can rent the infrastructure without having to forecast and provision for the highest possible demand in advance. In this approach, you are responsible for correctly configuring the rented infrastructure.
Software as a Service (SAAS)
In another approach, you can rent a service offered by the vendor and then configure the service by using the interface provided by the vendor, without having to know what infrastructure the vendor uses to provide that service. This approach is called Software as a Service (SaaS) because you pay to use defined services. For example, Microsoft Exchange Online carries a per-mailbox charge. To configure it, you use a web application supplied by the vendor to request mailboxes, and name and dimension them. You receive a password for that user and nothing else is necessary—users can access their mailboxes immediately.
Platform as a Service (PASS)
The third approach is Platform as a Service, or PaaS. In this approach, you rent a platform on which you deploy your applications without configuring the infrastructure and without the limitations of the SaaS approach.
The Windows Azure platform fits best in the PaaS category, because it doesn’t provide access to the underlying virtualization environment or operating system details such as the network interface, IP configuration, and disk management.
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