One of the two most common types of image formats used on the World Wide Web (the other being GIF). The shorter extension, JPG (without the E), is usually used in association with PCplatform files.
A standardized image compression mechanism, JPG is named after the original name of the committee that wrote the standard, the "Joint Photographic Experts Group." JPG is designed for compressing either full-color or gray-scale images of natural, real-world scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or line drawings. JPG handles only still images, but there is a related standard called MPEG for motion pictures. JPG is "lossy," meaning that the decompressed image isn't quite as sharp as the one you started with. (There are "lossless" image compression algorithms, but JPG achieves much greater compression than is possible with lossless methods.) JPG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness. Thus, JPG is intended for compressing images that will be looked at by humans. If you plan to machine-analyze your images, the small errors introduced by JPG may be a problem, even if they are invisible to the eye.
On the Web, there are image archives set up for Web designers to acquire graphics easily. These sites usually display thumbnail versions of images that may be GIFs or JPGs. Clicking on these thumbnails will usually link you to the larger, higher-resolution version of the same image.