There is no big difference between JPG and JPEG. The two are the recognized extension for Joint Photographic Expert Group and they are more or less serving the same purpose. At a time when Windows only save files with three letter extension, JPG is the most accepted format for saving images on the computer because MS-DOS could only read and handle extension files of three characters.
But now you can save your files with JPEG and it will be recognized because the new sets of operating system can now read and handle extension files of four characters. The difference is just in the letter E omitted in JPG. A perfect illustration is what we see in the word Honor and Honor, the two words mean the same thing but there's is an absence of letter 'U' in Honor. And the reason for JPG or JPEG extension is so that there can be a reduction of file size for easier storage.