Software as a Service (SaaS):

With the SaaS model, you consume as a service only the Applications that you need for your business. These applications run on the provider’s cloud infrastructure, making them accessible from various devices like browser or mobile.
The SaaS provider manages everything — that includes Infrastructure, Load balancers and firewalls, Operating Systems and runtime environments like .NET and Java, the line of business applications and services such as email or a CRM.

Platform as a Service (PaaS)

With the PaaS model, you get a core hosting operating system and optional building block services that allow you to run your own applications or third-party applications. You need not be concerned about lower level elements of Infrastructure, Network Topology, Security and Load Balancers — all this is done for you by the Cloud Service Provider. The Provider gives you a fully functional OS with major platform software.

Microsoft Windows Azure as PaaS can be used as a development, service hosting and service management environment. SQL Azure can provide data services, including a relational database, reporting and data synchronization. Both Windows Azure and SQL Azure are the key components of the Azure Cloud Platform. With this platform, you can focus on deploying your custom applications and can easily configure your applications to scale up or down as demands change.

Microsoft Azure platform as a PaaS can support different roles, such as Worker and Web. For example, you can run web applications with the Web Role, as well as host middle tier applications, such as Workflow, in the Worker Role. Similarly, SQL Azure provides Microsoft’s core relational database engine as a platform service.

One of the key benefits of PaaS is that you need not be concerned about the running OS or updates (service packs) and hardware upgrades. The Provider regularly patches your OS, updates platform features (such as the core .NET platform or SQL database engine) and updates hardware on demand to meet your demand.

Infrastructure as a Service (IaaS)

With the IaaS model, you can outsource the elements of infrastructure like Virtualization, Storage, Networking, Load Balancers and so on, to a Cloud Provider like Microsoft.
To deploy your applications to the Cloud, you have to install OS images and related application software on the cloud infrastructure. In this model, it’s your responsibility to patch/update/maintain the OS and any application software you install. The Cloud provider will typically bill you on computing power by the hour and the amount of resources allocated and consumed (as per its service level agreement (SLA).
For example, using Microsoft Windows Azure, you can set up new Windows Server and Linux virtual machines and adjust your usage as your requirements change. You only have to pay for the service that you use.

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