In a traditional, on-premises computing environment, it is the responsibility of your company and IT department to handle applications, software, data, operation systems, virtualization, servers, storage, and networking, among other computing requirements. Both IaaS and PaaS offer cloud service options designed to relieve this responsibility, in part, from the shoulders of your business and your IT employees. However, the manner in which IaaS and PaaS handle this is different.
IaaS stands for Infrastructure as a Service, and as you might guess, IaaS providers offer an IaaS platform that removes the burden of infrastructure concerns from your business. Rather than purchasing servers and other networking equipment, your business accesses these critical infrastructure resources through a third-party vendor. Your IT department would still be responsible for installing, maintaining, and updating any resources involving applications, data management, and operating systems, as well as taking responsibility for runtime and middleware concerns.
PaaS refers to Platform as a Service and goes a step further than IaaS in what it offers. PaaS providers can provide access to infrastructure resources such as networking, data storage, and servers, while also providing virtualization services that handle operating systems for virtual machines and middleware for developers and programmers to leverage in the course of completing daily operations. By utilizing a PaaS vendor, this leaves your in-house IT employees with the responsibility of applications and data management as part of their core duties.
In short, you can think of an IaaS solution as one of the cloud service models that is limited to storage, networking, and virtualization. An IaaS model is often provided on a pay-as-you-go basis, allowing you to increase or decrease resources as needed. PaaS solutions can vary in the robust nature of offerings based on PaaS providers, but in general, provide hardware and software tools that are accessible over the internet.