Software as a service

Software as a service is a software licensing and delivery model in which software is licensed on a subscription basis and is centrally hosted.
It is sometimes referred to as "on-demand software", and was formerly referred to as "software plus services" by Microsoft.

Example: 
Salesforce.com
Customer relationship management (CRM) software and cloud computing from the leader in CRM solutions for businesses large and small.
Amazon Web Services
Amazon Web Services offers reliable, scalable, and inexpensive cloud computing services. Free to join, pay only for what you use.