Initially developed by a company with the name of Gerber, Gerber format currently refers to a widely accepted standard PCB industry software sticks to, capable of describing circumstances of board images such as conductor layers, soldermask layers, legend layers.
Printed circuit boards are designed in a specialized EDA (Electronic Design Automation) or a CAD (Computer-Aided Design) system that further generate board manufacturing data based on which circuit board fabrications are commenced. PCB manufacturers won't fully understand all details of a PCB design file unless Gerber format file is contained in it as reference and guidelines. Gerber format file is applied to describe design requirement of each image of a circuit board and it can be applied for both bare board fabrication and PCB assembly.
When it comes to bare board fabrication, Gerber format is called for by both standard photoplotters and other manufacturing equipment desiring image data like legend printers, direct imagers or AOI (Automated/Automatic Optical Inspection) equipment etc. put it simply, Gerber format files have to be depended from beginning to the end of PCB fabrication process.
When it comes to PCB assembly, a stencil layer is included in Gerber format and component locations are regulated as well, which will be regarded as significant reference data for SMT (Surface Mount Technology) assembly, thru-hole assembly and mix of them.