The Gerber format is an open ASCII vector format for printed circuit board (PCB) designs.
It is the de facto standard used by PCB industry software to describe the printed circuit board images: copper layers, solder mask, legend, drill data, etc.

The official website contains the specification,test files, notes and the Reference Gerber Viewer to support users and especially developers of Gerber software.[1]

Gerber is used in PCB fabrication data.PCBs are designed on a specialized electronic design automation (EDA) or a computer-aided design (CAD) system.
The CAD systems output PCB fabrication data to allow fabrication of the board. This data typically contains a Gerber file for each image layer (copper layers, solder mask, legend or silk…). Gerber is also the standard image input format for all bare board fabrication equipment needing image data, such as photoplotters, legend printers, direct imagers or automated optical inspection (AOI) machines and for viewing reference images in different departments. For assembly the fabrication data contains the solder paste layers and the central locations of components to create the stencil and place and bond the components.

There are two major generations of Gerber format:

Extended Gerber, or RS-274X. This is the current Gerber format. In 2014, the graphics format was extended with the option to add meta-information to the graphics objects. Files with attributes are called X2 files; those without attributes are X1 files.[10][1]
Standard Gerber, or RS-274-D. This obsolete format was revoked.[11][12][13]
The standard file extension is .GBR or .gbr[1] though other extensions like .GB, .geb or .gerber are also used.