Web Design in a Nutshell, 3rd Edition

Lossless Versus Lossy Compression

Compression can be lossless, which means no information is lost and the final file is identical to the original.

Most compression schemes use forms of lossy compression. Lossy compression sacrifices some data from the file to achieve much higher compression rates. Lossy compression schemes, such as MPEG, use complicated algorithms that toss out data for sound and image detail that is not discernible to the human ear or eye. The decompressed file is extremely similar in character to the original, yet is not identical. This is similar to the way JPEG handles still images.