Cloud can be simplified as the combination of IaaS, PaaS, and SaaS.
IaaS: Infrastructure as a Service. Tenants maintain apps and VMs, and vendors maintain the hardware. Example: Amazon’s EC2, Microsoft’s Azure VMs, etc.
PaaS: Platform as a Service. Tenants focus on core business and maintain relevant core business apps only, and delegate the responsibilities of VMs and hardware to vendors. Tenants deploy apps and codes to a platform vendors provide. Example: Azure’s app service, Amazon’s Elastic Beanstalk
SaaS: Software as a Service. Tenants focus on core business and choose relevant vendors’ software offerings, and delegate the responsibilities of software, VMs, and hardware to vendors. Examples: Gmail, Sky Drive, Dropbox, SharePoint online, Office online, Salesforce, etc.
Table 1. Responsibilities of Tenants and Vendors