There are three main categories in Cloud computing they are Infrastructure as a Service, Software as a Service and Platform as a Service.

Infrastructure as a Service
Infrastructure as a Service is a provision model in which an organization outsources the equipment used to support operations, including storage, hardware, servers and networking components. The service provider owns the equipment and is responsible for hosting, running and maintaining it.

Software as a Service
Software as a Service 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. It is becoming an increasingly prevalent delivery model as underlying technologies that support Web services and service‐oriented architecture become increasingly available.

Platform as a Service
Platform as a Service is an outgrowth of Software as a Service. It is a way to rent hardware, operating systems, storage and network capacity over the Internet. The service delivery model allows the customer to rent virtualized servers and associated services for running existing applications or developing and testing new ones.