Short for Software as a Service, SaaS (pronounced "sass") is software that runs on multiple devices. Access to the software is provided as an ongoing service, rather than a product you purchase for a one-time cost. Most cloud services are categorized as SaaS.
The SaaS user interface may be a web app that you use in your web browser, or an app you download and run on the device. Or, the interface may be integrated with your operating system, as part of the Windows File Explorer, for example.
Benefits and downsides
The benefit of SaaS is that you don't need to own or manage the hardware that runs the software. The ability to access your software from any web browser, desktop computer, or mobile device offers synchronization, convenience of use, and freedom when choosing what device(s) you use. And, if something goes wrong, the service provider is (likely) responsible for resolving the problem, and contractually obliged to fix it.
One major downside is that if the service goes out, by service outage or discontinuance, your options are limited by your reliance on the service. And, because your data is hosted off-site (in the cloud), your data security relies on the trust you have in the service provider.
An example of a SaaS is Microsoft Office 365, a software suite hosted by Microsoft in its cloud. Users log into the Office 365 website and can access Microsoft Word, Excel, PowerPoint, and other programs from their browser.