IaaS , PaaS or SaaS

IaaS	PaaS	SaaS
An instant computing infrastructure, provisioned and managed over the Internet. IaaS enables you to quickly scale resources to meet demand and only pay for what you use..	A complete development and deployment environment in the cloud. With PaaS, you can build and deploy everything from simple cloud-based apps to sophisticated, cloud-enabled enterprise applications.	Allows users to connect to and use cloud-based apps over the Internet. Common examples are email, calendaring, and office tools such as Microsoft Office 365.
IaaS avoids the expense and complexity of buying and managing your own physical servers and other datacenter infrastructure.  Each resource is offered as a separate service component, and you rent the resource as long as you need it. As a result, IaaS is flexible	Like IaaS, PaaS includes infrastructure such as servers, storage, and networking. In addition, it also includes middleware, development tools, and other services.	SaaS provides a complete software solution that you purchase on a pay-as-you-go basis from a cloud service provider.
You can provision common infrastructure such as VMs, storage, virtual subnets, firewalls, and VPNs to build a solution. You don't need to manage physical servers and appliances. However, you are responsible for configuring and managing the components. For example, configuring firewalls, updating VM OS's, updating DBMS's, and runtimes.	PaaS removes the need to manage software licenses, middleware, and infrastructure of the services. You manage the applications and services you develop, and the cloud service provider typically manages everything else.	You can rent the use of an application for your organization. Your users connect to the service over the Internet, usually with a web browser. All of the underlying infrastructure, middleware, app software, and app data are located in the service provider’s data center. The service provider manages the hardware and software, and with the appropriate service agreement, will ensure the availability and the security of the app and your data as well. SaaS allows your organization to get quickly up and running with an app at minimal upfront cost.
When to use IaaS? Website hosting: If you want more control of hosting a website, running websites using IaaS may be a better option than traditional web hosting. Web apps: IaaS provides all the infrastructure to support web apps, including storage, web and application servers, and networking resources. Organizations can quickly deploy web apps on IaaS and easily scale infrastructure up and down when demand for the apps is unpredictable. Storage, backup, and recovery: Storage management can be complex, requiring a large capital investment and skilled staff to manage data and meet legal and compliance requirements. IaaS can help simplify planning, management, unpredictable demand, and steadily growing storage needs. High-performance computing: If you have a workload that requires high-performance computing, you can run the workload in the cloud avoiding the up-front cost of the hardware and only pay for the usage when needed. Big data analysis: If you have large data sets that contain potentially valuable patterns, trends, and associations, IaaS can provide the processing power to mine data sets to locate patterns.	When to use PaaS? Development framework: PaaS provides a framework that developers can build upon to develop or customize cloud-based applications. Similar to the way you create an Excel macro, PaaS lets developers create applications using built-in software components. Cloud features such as scalability, high-availability, and multi-tenant capability are included, reducing the amount of coding that developers must do. Analytics or business intelligence: Analysis tools provided as a service allows you to analyze and mine data. Organizations can find insights and patterns to predict outcomes to improve forecasting, product design decisions, investment returns, and other business decisions.	When to use SaaS? You can use a SaaS CRM provider to quickly implement a solution to your organization's sales team. For organizational use, you can rent productivity apps, such as email, collaboration, and calendaring; and sophisticated business applications such as customer relationship management (CRM), enterprise resource planning (ERP), and document management. You pay for the use of these apps by subscription or according to the level of use.
Advantages: Eliminates capital expense and reduces ongoing cost: IaaS sidesteps the upfront expense of setting up and managing an on-site datacenter, making it an economical option for start-ups and businesses testing new ideas. As soon as you’ve decided to launch a new product or initiative, the necessary computing infrastructure can be ready in minutes or hours, rather than the days or weeks—and sometimes months—it could take to set up internally. Improves business continuity and disaster recovery: Achieving high availability, business continuity, and disaster recovery is expensive, since it requires a significant amount of technology and staff. But with the right service level agreement (SLA) in place, IaaS can reduce this cost and access applications and data as usual during a disaster or outage. Respond quicker to shifting business conditions: IaaS enables you to quickly scale up resources to accommodate spikes in demand for your application— during the holidays, for example—then scale resources back down again when activity decreases to save money. Because you don’t need to first set up the infrastructure before you can develop and deliver apps, you can get them to users faster with IaaS. Increase stability, reliability, and supportability: With IaaS, there’s no need to maintain and upgrade hardware or troubleshoot equipment problems. With the appropriate agreement in place, the service provider assures that your infrastructure is reliable and meets SLAs.	Advantages: Reduced development time: PaaS development tools can reduce development time for new applications. Developers can use pre-coded application components built into the platform, such as workflow, directory services, security features, and search. Platform as a service components can give your development team new capabilities without you needing to add staff having the required skills. Develop for multiple platforms: Some service providers give you development options for multiple platforms, such as desktop, mobile devices, and browsers making cross-platform apps quicker and easier to develop. Use sophisticated tools affordably: A pay-as-you-go model makes it possible for individuals or organizations to use sophisticated development software and business intelligence and analytics tools that they could not afford to purchase outright. Support geographically distributed development teams: Because the development environment is accessed over the Internet, development teams can work together on projects even when team members are at remote locations. Efficiently manage the application lifecycle: PaaS provides all of the capabilities that you need to support the complete web application lifecycle: building, testing, deploying, managing, and updating within the same integrated environment.	Advantages Gain access to sophisticated applications: To provide SaaS apps to users, you don’t need to purchase, install, update, or maintain any hardware, middleware, or software. SaaS makes even sophisticated enterprise applications, such as ERP and CRM, affordable for organizations that lack the resources to buy, deploy, and manage the required infrastructure and software themselves. Pay only for what you use. You also save money because the SaaS service automatically scales up and down according to the level of usage. Use free client software: Users can run most SaaS apps directly from their web browser without needing to download and install any software. You don't need to purchase or deploy client software for your users. Access app data from anywhere: With data stored in the cloud, users can access their information from any Internet-connected computer or mobile device. And as app data is stored in the cloud, no data is lost if a user’s computer or device fails.

Source: https://azure.microsoft.com/en-au/overview/what-is-iaas/