There are indeed a wide variety of 3D scanners, it's a field of technology possibly more diverse than 3D printing.

The main technologies are:
laser tomography, which uses a laser or set of lasers and camera to detect contours. Most small turntable scanners use this.
Light Coding Or Stereoscopic scanning, which uses two (or more) cameras to get a 3D image or video. The Microsoft Kinect game system uses this, and many portrait and room scanners use this.
Photogrammetry, using regular photos of an object from many, many different angles under the same conditions to reconstruct the object as a computer model. A good (and free) example of this is Autodesk 123D Catch. Of course, since it's free, it takes a long time to process, and I'm pretty sure Autodesk gets to look in on your scans and use them if they want to.

Laser Tomography is great for scanning small things in high detail, Light Coding is great at scanning whole rooms or even whole building interiors, and Photogrammetry is great at taking portraits of living things (using many many small cameras in an array around the person or thing to take the pictures simultaneously)