What is JPEG file?

PEG (pronounced "jay-peg") is a standardized image compression mechanism. JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard. JPEG compresses either full-color or grayscale images, and works best with photographs and artwork. For geometric line drawings, lettering, cartoons, computer screenshots, and other images with flat color and sharp borders, the PNG and GIF image formats are usually preferable.

JPEG uses a lossy compression method, meaning that the decompressed image isn't quite the same as the original. (There are lossless image compression algorithms, but JPEG achieves much greater compression than is possible with lossless methods.) This method fools the eye by using the fact that people perceive small changes in color less accurately than small changes in brightness.