An algorithm for compressing bitmap images that contain smooth variations of tone and colour.
The JPEG algorithm is designed to discard visual detail that the eye cannot perceive, creating a smaller file but without noticeably altering the image.
The JPEG algorithm is optimized for photo-style images. It performs poorly when used on images with sharp edges of colour or tone, such as line art, text, logos, cartoons, and so on.
It is common to refer to a "JPEG file" when talking about an image file that uses JPEG compression. (Technically, the file format is either JFIF or Exif, but this detail is rarely important in practical use.)
JPEG stands for Joint Photographic Experts Group, the group which created the standard.
Also see: PNG, SVG.