It's a file extension for graphic files with the certain method
of compression. Usually jpg files take less disk space then, for
instance, tiff files. The extension JPG refers to JPEG files. The E
is omitted to fit it into the (deprecated) 8.3 file name
notation.
JPEG" stands for Joint Photographic Experts Group, the name of
the committee that created the image standard. The group was
organized in 1986, issuing a standard in 1992, which was approved
in 1994 as ISO 10918-1. JPEG is distinct from MPEG (Moving Picture
Experts Group), which produces compression schemes for video.
The JPEG file format is a compressed format, with some loss of
quality during compression. It is a popular web format due to the
generally smaller size of files compared to other formats. File
extensions are .jpg, .jpeg, and .jpe.