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

  IaaS PaaS SaaS
Apps  Tenants  Tenants  Vendors
VMs  Tenants  Vendors  Vendors
Hardware  Vendors  Vendors  Vendors