World File

A World file is an ASCII text file that is used to georeference raster images. The format was designed by ESRI. A world file contains 6 lines:


 * 1) x-dimension of a pixel in map units
 * 2) row rotation parameter
 * 3) column rotation parameter
 * 4) negative value of y-dimension of a pixel in map units
 * 5) x-coordinate of center of upper left pixel
 * 6) y-coordinate of center of upper left pixel

Naming conventions
According to this Wikipedia entry, the naming of a World file (including its extension) is based on the name of its corresponding raster image. This would make the automatic identification of World files rather difficult.

Disambiguation
ESRI (the creator of the World file format) also maintains another format that is used to define coordinate transformations in CAD datasets. Confusingly, they also refer to these files as 'World files', even though it is a completely different format. These 'World files' are typically saved with a '.wld' extension.

Links
No official specification appears to exist, however the following links both provide an adequate explanation of the format:


 * ESRI World File Format
 * FAQ: What is the format of the world file used for georeferencing images? (ESRI Knowledge Base)
 * Wikipedia: World file - Provides an elaborate description of the meaning of the parameters in a World file as well as a discussion of the naming/file extension conventions
 * Online World file calculator