While all three service models are cloud-based, there are several differences between them. IaaS is infrastructure, PaaS is platform, and SaaS is software. If you visualize a pyramid, IaaS would be the bottom layer (e.g. compute and storage), PaaS is the middle layer (e.g. app development, deployment, management), and SaaS is at the top (e.g. the actual software accessed by end users).