Cloud computing usually is classified in three categories: SaaS, PaaS, and IaaS.
SaaS: Software as a Service
SaaS is software that is centrally hosted and managed for the end customer. It usually is based on a multitenant architecture—a single version of the application is used for all customers. It can be scaled out to multiple instances to ensure the best performance in all locations. SaaS software typically is licensed through a monthly or annual subscription
PaaS: Platform as a Service
With PaaS, you deploy your application into an application-hosting environment provided by the cloud service vendor. The developer provides the application, and the PaaS vendor provides the ability to deploy and run it. This frees up developers from infrastructure management, allowing them to focus strictly on development.
IaaS: Infrastructure as a Service
An IaaS cloud vendor runs and manages server farms running virtualization software, enabling you to create VMs that run on the vendor’s infrastructure. Depending on the vendor, you can create a VM running Windows or Linux and install anything you want on it. Azure also provides the ability to set up virtual networks, load balancers, and storage and to use many other services that run on its infrastructure. You don’t have control over the hardware or virtualization software, but you do have control over most everything else. In fact, unlike PaaS, you are completely responsible for it.
Azure Virtual Machines, the Azure IaaS offering, is a popular choice when migrating services to Azure because it enables the ―lift and shift‖ model for migration. You can configure a VM similar to the infrastructure currently running your services in your datacenter and migrate your software to the new VM. You might need to make tweaks, such as URLs to other services or storage, but many applications can be migrated in this manner.