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A world file is a plain text, ASCII, file that contains the location, scale, and rotation of a raster image. The files are 6 lines and contains the following:

 

Line 1: pixel size in the x-direction in map units/pixel

Line 2: rotation about y-axis

Line 3: rotation about x-axis

Line 4: pixel size in the y-direction in map units, almost always negative

Line 5: x-coordinate of the center of the upper left pixel

Line 6: y-coordinate of the center of the upper left pixel

 

The name of the world file is based on the raster file's name. By convention the second letter of the image filename extension is removed, and the letter "w" is added at the end. For example, the world file for "raster.jpg" would be "raster.jgw". Most world files are made available with the raster image when it is purchased.

 

For more information on world files see the link below.

 

http://en.wikipedia.org/wiki/World_file