What is SaaS? Definition and explanations
SaaS (Software as a Service) is the cloud service model where the consumer uses the provider's applications running on a cloud infrastructure. Apps are accessed from various devices through a thin client (browser) or a program interface (NIST SP 800-145). The consumer does not manage network, servers, OS or storage — only, at most, limited application settings. Subscription is the usual commercial model, not a NIST criterion.
In one sentence
SaaS gives access to a provider's applications, via browser or API, without managing the infrastructure.