With Cloud as a Service (CaaS) offerings, the management of IT hardware moves from the company (on-premise) to a service provider (usually offsite). There are several variants which differ in the degree to which the underlying infrastructure and applications/services have to be managed by the customer.
Whereas with Infrastructure as a Service (IaaS) the applications, runtimes, security and databases still have to be managed, with Platform as a Service (PaaS) it is only the application. Both variants are primarily aimed at developers. Software (or Cloud) as a Service (SaaS), on the other hand, is mostly just for the end user.
All offerings allow for timely scalability, reduced IT administration efforts, the disappearance of investments in server hardware and more.