In a nutshell, Photosynthing is a method to combine multiple images from the same scene into an explorable panorama. The way it differs from normal panoramas is that the images do not need to have been taken from the same spot. In fact, it works best when images are taken from different view points.
Its a Microsoft project, and as yet isn't available to run on a Mac; to quote their web site,"Unfortunately, we're not cool enough to run on your OS". It goes on to say,"as soon as we have a Mac version ready, it will be up and available on our site". If you have a PC though, do check it out as it looks very interesting.