One of the original and most basic forms of 3D scanning is photogrammetry. This is the practice of taking many pictures of an object or subject and stitching the photos together and approximating the dimensions. This can be either a manual or automatic process - old Autodesk suites had software where you would manually define dimensions and object boundaries whereas newer applications (ex. AutoCatch 123D) will automatically attempt to guess the dimensions and create a printable model for you. This is one of the less precise and reliable methods of scanning.
Structured light scanning is where patterns are flashed onto a subject and the differences between the projected pattern and visible pattern are transcribed to very accurate dimensions. In short, patterns flash, and the computer figures out how you are shaped because the patterns are no longer straight. This does not record color information by itself, and is oftentimes supplemented by a photogrammetric scanner.