The Joint Photographic Experts Group (JPEG) format is a commonly-used lossy image format.
Common file extensions are .jpg, .jpeg, .JPG, .JPE, and .jfif.
JPEG files can contain lots of metadata in several formats: Exif, IPTC, GPS, Camera Raw, etc. The exif and jhead command tools can extract and manipulate some of that metadata. Adroit Photo Forensics can be used to extract, view and group metadata from jpeg and camera Raw files.