The Big Question—What is 3D Printing? - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 4

Two dimensional objects can be drawn in such a way that they look like they have
length, width, and height, but they're still flat. They still exist only in two dimen-
sions.
A 3D printer is a printer that prints in three dimensions. That's as simple a defini-
tion as you'll ever find. When your inkjet or laser printer prints out a square on a
piece of paper, all you have is a two-dimensional square on a flat piece of paper.
It lacks height. In fact, an advertisement for a 3D printer might state, “New and
improved! Now comes with HEIGHT!”
But if a 3D printer can add that third dimension, then it's possible to print that
square as a cube, as in Figure 1.3 .