Harry Miller who is a documentation writer on the VSTO and VSTA team has created a short video that looks at VSTA. This video does an excellent job at explaining at a high level (manager view) what is VSTA, what are some of the features and how it is integrated.
ShapeApp sample application.
ShapeApp macro recording.
ShapeApp macro generated code
I also want to thank Kathleen for working with Harry to post this on her blog.