3D printers build models in layers, which you can see if you look at a model closely. If you want a smoother, shinier surface, you can achieve injection mold-like quality with a little acetone ...

Sanding, bead blasting, and vapor smoothing are the most-popular methods for achieving smooth plastic 3D-printed parts.

The 'smoothing beautifier' will make your 3D prints less ugly Sculpteo's post-printing technique uses chemistry to smooth out objects.