Software as a Service - using software applications over the internet without installing them on your computer. Like using Netflix instead of buying DVDs - you access the service, not own the software.
Gmail, Salesforce, and Slack are SaaS applications you access through a web browser without installing anything.
SaaS is a cloud service model (not a single vendor service). AWS, Azure, GCP, and OCI primarily provide IaaS/PaaS platforms that can host SaaS products, and each also offers many SaaS applications (e.g., email, collaboration, CRM) through separate product lines or marketplaces rather than one named 'SaaS' service.