What's the Gerber?

The Gerber format is an open 2D binary vector image file format. It is the standard file used by printed circuit board (PCB) industry software to describe the printed circuit board images: copper layers, solder mask, legend, etc.


Gerber files should be inside a .rar or.zip archive with standard file extensions:
Extension                       Layer

• pcbname.GTL                 Top Copper 

• pcbname.GTS                 Top Soldermask 

• pcbname.GTO                 Top Silkscreen 

• 
  

• pcbname.GBL                 Bottom copper

• pcbname.GBS                 Bottom Soldermask: 

• pcbname.GBO                 Bottom Silkscreen: 

• pcbname.TXT                  Drills

• pcbname.GML/GKO        *Board Outline:

• 
  

• 4 layer board also need

• pcbname.GL2                   Inner Layer2

• pcbname.GL3                   Inner Layer3

Notes: Gerber file must be RS-274X format.  Drill file(pcbname.TXT) should be Excellon format. 
PCB Outline is required. Drawing board outline in Eagle (layer 20).

Gerber file is used for the manufacturing process. 

Below is a picture on how to compress the necessary files.