Hello Lee Ann,
If you access http://www.ariba.com under solutions you can select the 'Buy' link and the solution needed. On this screen to the right you will see an option for data sheets that will enable you to download documents that speak to the benefits of using each module (Sourcing, Contracts, P2P ect..).
Let me know if you were looking for something different. Thanks,
Robert Myroniuk
Ariba Consultant - Source to Pay (S2P)
Ariba, an SAP Company