Software as a Service (SaaS) is based on the idea that the software and the IT infrastructure are operated by an external cloud provider and can be used by customers and resellers.
For the usage of the software only an internet capable device and an internet connection to the cloud provider are needed. The access to the software is realised through a web browser.
The advantage of Software as a Service is that the customer or reseller can concentrate on his core business. Hardware, operation system, application, back up, monitoring, maintenance and standby duty are in the responsibility of the cloud provider.
More information can be found at operations.