A file format for images, optimized for photographic content (as opposed to diagrams, charts, etc.)
JPEG is one of the most common file formats for photos.
JPEG compresses image data so files are smaller and use less storage space. JPEG is a lossy format, meaning some amount of detail data may be permanently lost when creating the JPEG file. That loss is adjustable with JPEG at the time the JPEG file is created; the software that creates the JPEG file can apply more compression or less. More compression results in a smaller file with less visual detail. Less compression results in a larger file that has more of the original visual detail.