IaaS, as the name suggests, is a way of providing Cloud computing infrastructure such as virtual machines, storage drives, servers, operating systems & networks, which is also an on-demand service like that of SaaS. Rather than purchasing servers or developing software, clients buy those resources as a fully outsourced service based on their requirement. "Public cloud" is considered as an infrastructure that consists of shared resources, based on a self-service over the Internet. In one word, it is the only layer of the cloud where the customer gets the platform for their organization to outsource IT infrastructure on a pay-per-use basis.
