There is a myriad of materials you can use in 3D printing. Most desktop 3D printers, however, are limited to various plastics.
One of the first questions asked by people interested in 3D printing is: Where do I get things - to be more precise: models - for 3D printing?
So, what can a 3D printer be used for? What can I build with a 3D printer?
Transform yourself from newb to ninja with Chuck Hellebuyck's Beginner's Guide to 3D Printing.