It basically turns a liquid reservoir of material (plastic, resin, something like that) and hardens it using a laser acting against a platform. The laser shoots round in a preset pattern creating a hardened layer on the platform, then the platform drops by a tiny amount and the laser does another layer, and so on until you end up with a solid object. Most firms use them now instead of employing pattern or model makers, just program the printer and let it run overnight, next morning you've got your piece.
