The Gerber Format
All printed circuit boards (PCB) described on this site are defined in the Gerber format. This page provides a quick description of the format for those who are new to the concept.
The Gerber format was developed 30 years ago to control photoplotters. These draw an image on photosensitive film by focusing a spot of light on the film and moving the head around, just what you need to draw an image of a PCB for etching. Modern machines use lasers but the basic process is the same.
A file in the Gerber format contains the instructions for drawing one layer of the PCB. A modern double sided board has up to six layers (top copper, top solder mask, top silkscreen, bottom copper, etc). You also need a file to describe the drill holes so the total for a typical PCB can be up to seven files. Not all PCBs need each layer, for example, many designs omit the bottom silk screen, so in that case only six files are required.
These files (and one item of extra information described below) are all that you need to completely describe the PCB. This is great because you can electronically send the files to a PCB fabrication house and they should be able to make a perfect reproduction of your board. All PCBs on this site are defined by just their Gerber files.
The one extra item of information you need to provide to the fabricator is the format of the numbers used in the files. On this site all designs use "2:4 leading" and one way or another you need to tell the fabricator this important snippet. Some sites (such as BatchPCB) automatically assume this, others specifically ask you, or you could include a README.TXT file providing the information.
There is no fixed standard for naming the files so quite often you have to specify to the fabricator what each file represents, this can also be done in the readme file. The following list of extensions is becoming recognised as a de facto standard and is used in the designs on this site:
.GTL is the Top Copper
.GTS is the Top Soldermask
.GTO is the Top Silkscreen
.GBL is the Bottom Copper
.GBS is the Bottom Soldermask
.GBO is the Bottom Silkscreen (not often used)
.TXT is the Excellon Drill File (commands for the numerically controlled drilling machine)
These files are normally zipped up and sent as a single zip file to a PCB house for fabrication. Some fabricators charge more for items such as silk screening and the soldermask so you can leave those files out to reduce the cost.