JPG and JPEG are both popular image formats, and the only difference between them is what you see. Older Windows versions did not allow file extensions with more than three characters, so they used .jpg instead of .jpeg, which was commonly used on Mac and UNIX systems.
Even though Windows computers now support files with the longer .jpeg extension, the three-character extension has become more widely used. Apart from the extra character, there is no real difference between JPG and JPEG images.