Software as a Service

With SaaS, you’re essentially renting or using a fully developed application. Email, financial software, messaging application.
SaaS model may be the least flexible, it’s also the easiest to get up and running.
It requires the least amount of technical knowledge or expertise to fully employ.

Shared Responsibility Model

SaaS places most of the responsibility with the cloud provider and the least responsibility with the user.
In a SaaS environment you’re responsible for the data that you put into the system, the devices that you allow to connect to the system, and the users that have access.
The cloud provider is responsible for everything which includes physical security of the datacenters, power, network connectivity, and application development and patching.