Bart Van der Scheuren, of the Belgian 3D printing company Materialise, does a good job of explaining how the three main 3D printing technologies work.
: Fused-deposition modeling : uses an nozzle to lay down the part one-layer-at-a-time using plastic or other media
: Laser sintering : uses a laser (or in some printers a fine spray of glue) to fuse powder into solid shapes
: Stereolithography : uses a laser is used to fuse a very special liquid into plastic.