OBA (Office Business Applications) are those applications that use Microsoft Office as the front end of various line of business applications.
OBAs connect Line of Business (LOB) systems with the people that use them through the familiar user interface of Microsoft Office. OBAs enable businesses to extend the Microsoft Office clients and servers into business processes running in LOB applications such as Enterprise Resource Planning (ERP), Customer Relationship Management (CRM), and Supply Chain Management (SCM). This enables enterprises to create new value from existing IT investments by combining them in innovative ways.
The best known OBA is Duet, jointly developed by Microsoft and SAP. OBA's offer a big advantage to ISV's to integrate their data on the desktop.
Mike Walker's Ramblings About Industry Architecture is the place to turn for more information about all things OBA. Walker's "What are OBAs?" post provides an overview of OBA and how it connects commercial off the shelf solutions to Microsoft Office in a composite application. And in his What types of OBAs are there?" post, Walker explains the difference between Office-client-only and Office-client-plus SharePoint OBAs.
His most recent post, MOSS 2007 Resources and Tools for building OBAs provides a list of tools for administrators and developers to build OBA apps on Microsoft Office SharePoint 2007.