Photosynth collections start with a set of photographs of roughly the same subject, such as a place, object, or monument. The photographs might have been taken all at once by the same photographer, or they might be a disparate set of pictures collected from different photographers at different times. The images are then processed by a preprocessor program that identifies “features” in the photographs – identifiable points in each image. (This picture shows a photo with some of its feature points superimposed.)
Microsoft Photography Blog : Photosynth Technology Preview
Video of Notre Dame and how Photosynth works here. The concept is to be able to check the ambience of a restaurant via photos accumulated and placed relative to each other to create a 3D world. Interesting idea, I think.