Infrastructure as a Service (IaaS) is a service model in the field of cloud computing. It means that clouds provide access to the use of computer-based resources (such as storage, hardware, and other infrastructure components).
Amazon pioneered this field with the introduction of Amazon Web Services (AWS) and its Amazon EC2 product in 2006.
IaaS is just one of many service models. Other service models include SaaS, PaaS, and FaaS.