Compression
From The Glossary of Digital Photography
The reduction in size of a file or image. Compression helps to conserve file space and makes data transfer faster. It is generally classified either as lossless or lossy. Lossless means that no data is lost and the reconstructed image is identical to the original. TIFF-LZW is an example of a lossless compression technique. With lossy compression, similar data is thrown away when the image is saved. When the image is reopened, it is similar, but not exactly the same as the original. JPEG is the most popular format for lossy compression. The more compression is used and the more times the image is resaved, the more data is lost. At some point, the image will degrade sufficiently to be noticed. The best advice is to keep the original in either a lossless compression file or a non-compressed file. Any compression should be the final process before delivery or final use.
