Infrastructure as a Service
IaaS gives you building blocks such as virtual machines, networks, and disks. The provider operates the physical hardware. You manage your operating system, applications, and configuration.
Think of renting an empty workshop: the building is ready, but you choose and maintain what goes inside.
Platform as a Service
PaaS provides a managed environment for running your code. The provider handles more of the underlying system, while you focus on application code, data, and configuration.
A managed application hosting platform is a common example. You deploy your application without directly maintaining its operating system.
Software as a Service
SaaS is a complete application you access as a user, such as browser-based email or a collaboration tool. The provider operates the application and its infrastructure.
IaaS offers more control, PaaS reduces infrastructure work, and SaaS provides a ready-to-use application. In every model, you still need to manage your access, data, and account settings.