CAM
Gerber (RS-274X)
In plain English
A set of picture files that show the factory exactly what each layer of the board should look like — like a stack of blueprints, one per floor of a building.
The technical version
Gerber RS-274X (extended Gerber) is the de facto standard image format for PCB manufacturing. Each layer is a separate file describing copper, mask, and legend geometry through aperture-based draws and flashes. Because it is purely image data, netlist and drill information must travel as separate files.