A Gerber file is simply an image of how one layer of your circuit design looks. Your circuit board consists of copper traces to connect the components together, silk-screen to show component values etc. and solder mask to prevent solder bridges between your traces.
Each of these layers can be represented by a Gerber file. If you open one in a Gerber viewer, you will be able to see how that particular layer looks. These pictures are used as a universal way of telling a machine how to create the electronic circuit board you have designed.
The great thing about the Gerbers is that they make you independent of a particular PCB design software!
At the time of writing (2012), the format you use for Gerber files is called RS-274X. It consists of commands and coordinates for drawing lines and predefined shapes.
The format is open and written in human-readable text. This means you can open a file in a text-editor and see the commands. The commands can be a bit cryptic though..
One thing to note is that the RS-274X format can specify holes, but usually, the Excellon format is used for holes instead.
Each Gerber file represents only one PCB layer. That means you will usually get several files for a board. The Gerber format represents images of your circuit board layers that let you choose whatever PCB software you want to use. The format used is called RS-274X. And that is all you really need to know.
Both expensive professional PCB software and free open-source PCB software will create Gerber files from your design that you can send to any PCB manufacturer to get your board produced.
In conclusion, Gerber files are:
images of your PCB
gives you freedom to choose PCB software
use the RS-274X format
An image of your PCB
