Everyone talks about 3D printing as though it is a universal technology when, in fact, it is a family of technologies. While all processes build an object layer by layer - why it's called additive manufacturing - they use different materials and techniques.
If, for example, you want a 3D printed plastic, you can choose from HP Jet Fusion (Nylon), Selective Laser Sintering (Nylon), Fused Deposition Modeling (ABS), PolyJet (Acrylic and Polypropylene-like) or Large-Format printing (Epoxy-infused Acrylic).
For metals, you can choose from Direct Metal Laser Sintering (Aluminum, Stainless Steel, Tool Steel and Titanium) or Printed Metal (Stainless Steel Bronze Alloy).
And if you want color, there's the ColorJet which prints in full-color composite.