Simply put, "Photosynth" can take a large number of photos of the same location, analyze their similarities, and use this to create a virtual 3D world simulated from 2D photos.
For example, a famous tourist attraction is visited by tens of thousands of people every day, and the internet sees a daily increase in tourist photos of that attraction. Photosynth collects these photos, correlates them with each other, and reorganizes them into a virtual 3D model of the attraction.