JPEG is a lossy image format. This means that in order to reduce size of image files some of the image information is discarded during the compression process. The higher the compression the more data is lost. This loss of quality in JPEG images appears as "blocking" and "ringing" artifacts, often appearing as "ghost shadows" along edges. A typical image editor, such as Adobe Photoshop, uncompresses a JPEG file for image editing and then recompresses it for writing back to the file. These recompression losses add up and image quality degrades further each time the file is opened and closed.
However, JPEG images consist of a number of independently encoded blocks typically 8x8, 8x16 or 16x16 pixels in size. There is a smart way to handle these blocks individually without having to uncompress and then recompress the untouched ones. Better JPEG takes advantage of this smart block handling, allowing for partial image editing without recompression of the entire image. This editing includes red eye removal, data/text imprinting, copy/paste and so on.