Hi, thanks for your questions. ArcGIS Online is not a SaaS version of desktop, it provides organizations with a cloud-based, collaborative content management system for maps, apps, data, and other geographic information. It's about making the organizations GIS, maps and data available to everyone. With it you can make and use the organizations maps and data in your work. Think of it as empowering everyone to participate.
One good place to start with what it can do for an organization is with the detailed help system and videos. Your organization can also participate in the beta program to see first hand what it can do. Start here with the help system:
http://help.arcgis.com/en/arcgisonline/help/010q/010q0000007m000000.htm
The ArcGIS Online blog is also full of good info:
http://blogs.esri.com/esri/arcgis/
-Paul-