Infrastructure as a Service (IaaS) is a cloud model which allows organizations to outsource computing equipment and resources such as servers, storage, networking as well as services, such as load balancing and content delivery networks. The IaaS provider owns and maintains the equipment while the organization rents out the specific services it needs, usually on a "pay as you go" basis.
Platform as a Service (PaaS) is a cloud model designed for software developers that streamlines the development process by shifting specific aspects of systems management to the service provider. PaaS is used to develop web and mobile applications using components that are pre-configured and maintained by the service provider, including programming languages, application servers and databases.
Software as a Service (SaaS) is a cloud model that delivers on-demand applications that are hosted and managed by the service provider and typically paid for on a subscription basis. SaaS solutions offer a number of advantages over on-premises deployments, including minimal administration and maintenance, anywhere access, and in many cases improved communication and collaboration.