A cloud computing infrastructure type where applications are hosted over a network (typically Internet) eliminating the need to install and run the software on the customer's own computers is known as Software as a Service (SaaS).
These are applications hosted over the internet so the user does not have to download them. An example is Google Apps, where you can access something similar to Microsoft Word or Excel, without having to download anything.
Software as a service (SaaS ) is a software licensing and delivery model in which software is licensed on a subscription basis and is centrally hosted. It is sometimes referred to as "on-demand software", and was formerly referred to as "software plus services" by Microsoft. SaaS is typically accessed by users using a thin client via a web browser. SaaS has become a common delivery model for many business applications, including office software, messaging software, …