JPG vs. JPEG image formats

Question 1

JPG and JPEG stand both for an image format proposed and supported by the Joint Photographic Experts Group. The two terms have the same meaning and are interchangeable.

To read on, check out Difference between JPG and JPEG.

The reason for the different file extensions dates back to the early versions of Windows. The original file extension for the Joint Photographic Expert Group File Format was ‘.jpeg’; however in Windows all files required a three letter file extension. So, the file extension was shortened to ‘.jpg’. However, Macintosh was not limited to three letter file extensions, so Mac users used ‘.jpeg’. Eventually, with upgrades Windows also began to accept ‘.jpeg’. However, many users were already used to ‘.jpg’, so both the three letter file extension and the four letter extension began to be commonly used, and still is.
Today, the most commonly accepted and used form is the ‘.jpg’, as many users were Windows users. Imaging applications, such as Adobe Photoshop, save all JPEG files with a ".jpg" extension on both Mac and Windows, in an attempt to avoid confusion. The Joint Photographic Expert Group File Format can also be saved with the upper-case ‘.JPEG’ and ‘.JPG’ file extensions, which are less common, but also accepted.

Question 2

The term "JPEG" is an acronym for the Joint Photographic Experts Group, which created the standard. .jpeg and .jpg files are identical. JPEG images are identified with 6 different standard file name extensions:

.jpg
.jpeg
.jpe
.jif
.jfif
.jfi

The jpg was used in Microsoft Operating Systems when they only supported 3 chars-extensions.

The JPEG File Interchange Format (JFIF - last three extensions in my list) is an image file format standard for exchanging JPEG encoded files compliant with the JPEG Interchange Format (JIF) standard, solving some of JIF's limitations in regard. Image data in JFIF files is compressed using the techniques in the JPEG standard, hence JFIF is sometimes referred to as "JPEG/JFIF".

Question 3

No difference at all.

I personally prefer having 3 letters extensions, but you might prefer having the full name.
It's pure aestetics (personal taste), nothing else.
The format doesn't change.

You can rename the jpeg files into jpg (or vice versa) an nothing changes: they will open in your picture viewer.

By opening both a JPG and a JPEG file with an hex editor, you will notice that they share the very same heading information.

Question 4

They are identical. JPG is simply a holdover from the days of DOS when file extensions were required to be 3 characters long. You can find out more information about the JPEG standard here. A question very similar to this one was asked over at SuperUser, where the accepted answer should give you some more detailed information.

Question 5

There is no difference between them, it just a file extension for image/jpeg mime type. In fact file extension for image/jpeg is .jpg, .jpeg, .jpe .jif, .jfif, .jfi

Question 6

There's no difference between the file extensions, and they are used interchangeably. I guess the 3-letter version stems from the DOS era...

However, there are different "flavors" of JPEG files. Most notably the JFIF standard and the EXIF standard. Most often these just use .jpg or .jpeg as file extensions, JFIF sometimes uses .jif or .jfif.