Simple, a 3D Printer grows layer after layer in very small slices of 6 ten thousands of an inch .0006″ and has the ability to deposit material with excellent precision.
Lets be clear; 3D Printing is Rapid Prototyping, but not all Rapid Prototyping systems are 3D Printers. A printer type rapid prototype system “jets” liquid material out of spray nozzles located in a block, laying down a 2-Dimensional bitmap, one on top of another at specified mechanical heights, or jets liquid out of the block into a material like a powder that absorbs the liquid and solidfies the powder into a hard bitmap image.
Source: www.kaiser3d.com