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Cloud computing usually is classified in three categories: SaaS, PaaS, and IaaS. However, as the cloud matures, the distinction among these is being eroded.


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.

Microsoft Office 365 is a prototypical model of a SaaS offering. Subscribers pay a monthly or annual subscription fee, and they get Exchange as a Service (online and/or desktop Outlook), Storage as a Service (OneDrive), and the rest of the Microsoft Office Suite (online, the desktop version, or both). Subscribers are always provided the most recent version. This essentially allows you to have a Microsoft Exchange server without having to purchase a server and install and support Exchange—the Exchange server is managed for you, including software patches and updates. Compared to installing and upgrading Office every year, this is much less expensive and requires much less effort to keep updated.

Other examples of SaaS include Dropbox, WordPress, and Amazon Kindle.


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 developers from infrastructure management, allowing them to focus strictly on development.

Azure provides several PaaS compute offerings, including the Web Apps feature in Azure App Service and Azure Cloud Services (web and worker roles). In either case, developers have multiple ways to deploy their application without knowing anything about the nuts and bolts supporting it. Developers don’t have to create VMs, use Remote Desktop Protocol (RDP) to log into each one, and install the application. They just hit a button (or pretty close to it), and the tools provided by Microsoft provision the VMs and then deploy and install the application on them.


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 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 almost 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.

Azure VM Scale Sets (VMSS) is built on top of Azure Virtual Machines and provides an easy way to deploy clusters of identical VMs. VMSS also supports autoscaling so that new VMs can be deployed automatically when required. This makes VMSS an ideal platform to host higher-level microservice compute clusters such as for Azure Service Fabric and the Azure Container Service.


Azure services
Azure includes many services in its cloud computing platform. Let’s talk about a few of them.
 Compute services This includes the Azure Virtual Machines—both Linux and Windows, Cloud Services, App Services (Web Apps, Mobile Apps, Logic Apps, API Apps, and Function Apps), Batch (for large-scale parallel and batch compute jobs), RemoteApp, Service Fabric, and the Azure Container Service.

 Data services This includes Microsoft Azure Storage (comprised of the Blob, Queue, Table, and Azure Files services), Azure SQL Database, DocumentDB, StorSimple, and the Redis Cache.

 Application services This includes services that you can use to help build and operate your applications, such as Azure Active Directory (Azure AD), Service Bus for connecting distributed systems, HDInsight for processing big data, Azure Scheduler, and Azure Media Services.

 Network services This includes Azure features such as Virtual Networks, ExpressRoute, Azure DNS, Azure Traffic Manager, and the Azure Content Delivery Network.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition