The Gerber format is an open ASCII vector format for 2D binary images. It is the de facto standard used by printed circuit board (PCB) industry software to describe the printed circuit board images. 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 PCB fabrication data to allow fabrication. Fabrication data contains a Gerber file for each image layer (copper layers, solder mask, legend or silk...). Drilled hole information is also transferred in Gerber but for historic reasons the Excellon format is often used. 

Typically, all these files are "zipped" into a single archive that is sent to the PCB bare board fabrication shop. The fabricator loads them into a computer-aided manufacturing (CAM) system to prepare data for each step of the PCB production process.

In PCB Gerber file, the following layers are necessary, you can find a template to download on right side.

Projectname.GTL: Top Layer

Projectname.GL2: Inner Layer(for four layer PCB)

Projectname.GL3: Inner Layer(for four layer PCB)

Projectname.GBL  Bottom Layer

Projectname.GTS  Top Solder Mask

Projectname.GBS  Bottom Solder Mask

Projectname.GTO  Top Silk

Projectname.GBO  Bottom Silk

Projectname.GKO/GML: Board Outline

Projectname.TXT  Drill Drawing

Projectname.GTP: Top Solder paste(Not necessary for PCB fabrication)

Projectname.GBP:  Bottom Solder Paste(Not necessary for PCB fabrication)

Notes:

1. Gerber file must be RS-274x format, you could download the template in right side.

2. Drill file(Projectname.TXT) should be Excellon format.

3. Gerber file and Drill file(Projectname.TXT) must be put in same file, or it will be easy to be ignored.

4. Projectname.GTP/GBP are not necessary layer for PCB manufacturing, but it is necessary for making stencil.

5. Gerber exported by Software with different suffix, all of them work, the suffixes mention in this article is the most common one and much easier to be recognized by our gerber view.

The following picture is gerber template review: