The stepper conducts light exposure through a mask to project the circuit’s image, much like a negative image in standard photography. Here’s how it works: The stepper is loaded with silicon wafers with a photo-resistive coating. A robotic arm grabs a wafer and sets it on a wafer stage. The wafer is aligned, and then light (perhaps extreme ultraviolet) is cast through a quartz plate with a chrome pattern of the circuits that will be created.
This process hardens a photo-resistive layer on the printed circuit board (PCB) or silicon wafer. The hardened areas stay behind in the form of circuit paths of PCBs and CPUs. Unexposed areas are then dissolved away by a solution bath, typically an acid in wet methods or plasma-like oxygen ions in dry methods.
See a simple demonstration of photolithography from the "Too Small to See" exhibition at the Cornell Nanotechnology Facility: