Is that individual layers are built up until the three dimensional shape is finished. It is a stack of thin slices. Think of a block of paper sheets carved up into a piece of sculpture, except the "printer" does it by laying down a sheet at a time. Sort of like the ancient Chinese art of lacquer sculpture--multiple coats of different colored lacquers are applied then allowed to dry and then carved to make the 3D figure, tomography in reverse.

Of course, if the thing you are making has moving parts, then each part must made separately and the collection of parts must be assembled by hand later. So you don't really make a 3D gun; you make a 3D barrel, a 3D receiver, a 3D trigger, and so on.

Read Vonnegut's book "Player Piano" to understand the motivation, if not the actual process. Instead of mechanical parts like a connecting rod or a piston being forged, cast, stamped or machined by highly paid craftsmen, one lonely pencil-necked geek programs a computer once to fabricate as many of the damned things as you want.

Its progress, if it is used to make more and better things cheaper and faster. But I suspect things will be designed so they can be fabricated by mindless minimum wage automatons, and quality (and the dignity of labor) will be compromised as a result. Of course, the pencil-necked geek, and his boss, make out like a bandit.

Its sort of how MacDonalds automates and systematizes the culinary process to allow skilled chefs to make cheaper, tastier, and more nutritious burgers. It is certainly possible for technical advances to put out a better product at a cheaper price, while simultaneously making the workplace more humane and civilized. Sometimes it even turns out that way. But it ain't necessarily so, and it is only rarely planned like that from the start.