Software as a Service – SaaS – or Cloud Computing is an online service where the software is not running on hardware at the enduser, but on a machine farm at an application service provider or directly at the software vendor.
The software licences are not purchased, but the software use is rented. For a certain period of time, for a certain project – there are several contractual options.
Software updates and hardware services are fulfilled by the service provider without overloading the IT department of the enduser.
Sometimes a small client utility is used to connect to the service, sometimes it runs over the HTTP or HTTPS protocols in a regular webbrowser.
Autodesk is currently running a pilot with the products: AutoCAD, Inventor and Revit op Autodesk Labs – Project Twitch.
ESRI has ArcGIS Online that provides ArcGIS users with a set of core services over the web.