Also known as Software as a Service is a business and deployment model that allows a provider to sell their application as a subscription service. SaaS providers have a single product that is used by many customers and typically hosted in the cloud. This means that while customer data is co-located together, data is not shared. SaaS companies are able to achieve massive economies of scale by being able to re-use their application & infrastructure to support many customers. An example of a SaaS company is Salesforce. They have thousands of customers using their application online, all storing customer data. Salesforce doesn’t give each customer their own copy of the software. Everyone is using the same software, but their data is stored intelligently and securely so that one application can support multiple customers.
in other words
The reason why you don't have to download a bunch of programs on your computer anymore.