JPEG
From The Glossary of Digital Photography
One of the most common file compression techniques for images currently in use. Although JPEG is actually a compression technique using two different image formats, it is widely used as a descriptor for a file format. The actual file formats are JPEG/JFIF, which is used mainly on the Web, and JPEG/Exif, which is commonly used by digital cameras that produce JPEG images. Many software applications can read both, so the difference between the two is transparent to the user most of the time. To add to the confusion, both formats use .jpg as the file extension. JPEG is a lossy file compression technique. It uses a technique known as discrete cosine transform to do the compression. Great reductions in image size are made possible by throwing away similar pixels. When the image is then uncompressed for display, it looks approximately the same as the original. How similar it looks is greatly dependent on how much compression is used. When using Photoshop, there are quality settings of from 1 to 12, with 12 being the highest quality and lowest compression. Image qualities of 10 to 12 are very high quality and there are few differences within this range. Those levels are generally safe for transmitting images to clients. For web use, high compression can be used, which will reduce storage requirements and increase downloading speed. While JPEG files can be opened any number of times without a problem, because of its lossy nature, repeated resaving of the image will degrade the quality. Higher compression levels will cause it to degrade quickly. See also JFIF; Exchangeable image file format (Exif or EXIF)
