There really isn’t a difference between a .TIF and a .TIFF, or a .JPG and a .JPEG, except that one uses an extension with three letters, the other with four. The files are the same, and will remain the same even if you change the file from the three-letter extension to the four-letter extension. For example, if you change a .TIF to .TIFF, the file will still open up the same; the only difference will be in the file name.
The two different file extensions for the same file type derived from Windows, which required files to have a file extension of no more than three letters. Thus, the original .TIFF files were abbreviated to .TIF to meet this requirement. Macintosh platforms, however, did not limit the file extensions to three letters, since the longer the extension meant the more descriptive. This is why Macs use file types with four extensions.