SPI is an acronym for the most common cloud computing service models, Software as a Service, Platform as a Service and Infrastructure as a Service.
Software as a Service (SaaS) is a software distribution model in which applications are hosted by a vendor or service provider and made available to customers over a network, typically the Internet.
Platform as a Service (PaaS) is a paradigm for delivering operating systems and associated services over the Internet without downloads or installation.
Infrastructure as a Service (IaaS) involves outsourcing the equipment used to support operations, including storage, hardware, servers and networking components.
The increasing selection of services delivered over the Internet is sometimes referred to as XaaS.