World files have the extension of the first and last letters of the image extension followed by a W. So for a TIF file, the world file would have the extension TFW. For a JPG, the world file would have the extension JGW.
 
A World file is a simple text file with 6 lines. You can create your own file in Notepad. To do this, open Notepad and enter the following values:

x resolution    <--the number of real world units per pixel in the X direction
amount of translation  <--usually 0 or a very small number
amount of rotation   <--usually 0 or a very small number
negative of the y resolution  <--the negative of the number of real world units per pixel in the Y direction
xMin coordinate (the real world Easting or longitude of the upper left corner)
yMax coordinate (the real world Northing or latitude of the upper left corner)

The translation and rotation terms are usually 0 or some very small number. Save the file as a *.xxW (TFW, JGW, PGW, etc) to the same folder as the image file, with the same file name as the image file.

Note that world files just contain the XY coordinates, not any projection information. More information can be found by doing a Google search, or looking on Wikipedia.

Updated: April 30, 2018