Three service models of cloud computing: IaaS, PaaS and SaaS
Infrastructure (infrastructure)-as-a-Service, Platform (platform)-as-a-Service, Software (software)-as-a-Service. The infrastructure is at the bottom, the platform is in the middle, and the software is at the top.
IaaS: Infrastructure-as-a-Service is the first layer.
PaaS: Platform-as-a-Service (Platform as a Service) The second layer is the so-called PaaS, sometimes called middleware.
SaaS: Software-as-a-Service is the third layer.
saas (CRM, Email, Virtual Desktop, Unified Communications, Online Gaming)
paas (database, development tools, web server, software operating environment)
iaas (virtual machine, server, storage space, network bandwidth, security protection)