Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as Service (SaaS) are the three main categories of cloud computing service models. Each type of cloud computing provides organizations and individuals with fully managed resources over the public internet—from storage and virtualization to hardware and software to applications. The difference between them is which resources you manage and which are managed for you.

“As a Service” generally refers to a cloud computing service that is fully managed by a third-party cloud service provider. Understanding what you want to manage (and what you don’t) is one of the most important steps on your journey to the cloud. 

Based on the service type you choose, the service provider is responsible for managing different elements in your computing stack:

• IaaS: The service provider gives you on-demand access to infrastructure services, including compute, storage, networking, and virtualization. You manage everything else—the virtual machines, operating systems, middleware, apps, and your data—but there is no need to maintain or update your own data center infrastructure. 
• PaaS: The service provider delivers and manages all the hardware and software resources needed for application development. You write the code and manage all the apps and data, but you do not have to manage or maintain the software development platform. PaaS manages more resources higher up the “stack” to further reduce the operational burden on developers and IT operations teams.
• SaaS: The service provider delivers and manages the entire application stack—from the hardware infrastructure all the way to the application itself—through the internet. All updates, bug fixes, and other general maintenance to all components are handled by the provider. All you have to do is connect to the app.