From the ESRI FAQ page:

Question

What is the format of the world file used for georeferencing images? 

Answer

The world file is an ASCII text file associated with an image and contains the following lines:

Line 1: x-dimension of a pixel in map units
Line 2: rotation parameter
Line 3: rotation parameter
Line 4: NEGATIVE of y-dimension of a pixel in map units
Line 5: x-coordinate of center of upper left pixel
Line 6: y-coordinate of center of upper left pixel

If your image lacks a world file, you can create your own world file, using a text editor. This is generally practical only when the image does not require any rotation or rectification to be properly georeferenced, meaning lines 2 and 3 should be zero. Line 4 is negative to convert from image row numbering (increasing from the top down) to map coordinates (increasing from the bottom up). For a rectified image, line 4 must be equal to line 1 and of opposite sign. The ArcInfo commands REGISTER and RECTIFY, as well as GRIDIMAGE and CONVERTIMAGE create a world file.

The world file must follow this naming convention: If the image file name has a 3-character extension (image1.tif), the world file has the same name followed by an extension containing the first and last letters of the image's extension and ending with a 'w' (image1.tfw). If the extension has more or less than 3 characters, including no extension at all, then the world file name is formed by simply appending a 'w' to the image file name.

             1.000000000000000
             0.000000000000000
             0.000000000000000
            -1.000000000000000
        672849.079999999960000
       3424501.250000000000000

1 meter resolution. Always leave lines 2 and 3 as zero.