PhotoScan is a type of software that uses photogrammetry to create 3D models of objects. Photogrammetry is the use of photography in the measuring of distances between objects. By taking multiple pictures of the object you wish to create a 3D model of, PhotoScan is able to look at the areas where the photos overlap to produce three-dimensional coordinates of the points of interest, much like the way your eyes can gauge distance, and using this information, a 3D model can be created.
Help with PhotoScan: Pt. 1, Pt. 2, Pt. 3
These videos created by Samantha Porter, a PhD candidate in Anthropology at the University of Minnesota, are very helpful with understanding how to operate PhotoScan and walk you through the process of making a 3D model using photogrammetric methods.