Gerbv is a small tool to view files of the Gerber file format. A format used by PCB manufacturers to describe the different layers of the design. It is basically a vary simple vectorized image format. The files are in ASCII format and possible to poke around in by hand.
A set of "apertures" are defined - possible shapes include: round, square, other polygons, as well thermal relieved pads. These apertures are then either flashed at specific coordinates, giving rise to a image of their exact shape, or dragged from one coordinate to another, giving rise to a line of the corresponding width. There is also a command to specify the outlines of a filled polygon.
A gerber file can contain multiple layers (not to be confused with PCB layers). Layers can either add or subtract from previous layers. This allows using simpler shapes to remove only parts of a filled area, e.g. clearing a blank hole within a polygon shape.
The complete file format specification is only 62 pages long and recommended reading for the interested. It is well written and easy to follow, and essential for some fun gerber file hacks.
sudo apt-get install gerbv