VSTO – Visual Studio Tools for Office
VSTA – Visual Studio Tools for Applications – similar to VBA (Visual Basic for Applications), although there is no plan yet to phase out VBA.
Note that the Microsoft Trinity team is responsible for both VSTO and VSTA.
VSTO V3 is in the pipeline – providing support for all office apps and server programming, but will not be out with Office 2007.
Instead, VSTO Cyprus will apparently be release with Office 2007. This permits creation of plug ins to ribbons, custom panes and form regions etc. The beta of this is apparently available now.
RAD – Rapid Application Development