JPEG Files can contain many things besides the image:
miniature of the image,
software setting which created the file,
EXIF settings of the camera (focal, exposure...).
The digital cameras especially add 10 kilobytes to each JPEG file.
If this information can be useful to manage locally its photographs, they are useless on a website or when sending the images by e-mail!
PureJPEG allows to remove this superfluous information, without any quality loss. PureJPEG runs on command line: purejpeg monimage.jpg