Lossy compression

compression of graphical data in which some of the information is lost and the quality of the picture is damaged although the compression is very efficient

A lossy compression method is one where compressing data and then decompressing it retrieves data that may well be different from the original, but is close enough to be useful in some way. Lossy compression is most commonly used to compress multimedia data (audio, video, still images), especially in applications such as streaming media and internet telephony. On the other hand lossless compression is preferred for text and data files, such as bank records, text articles, etc. Most lossy compression formats suffer from generation loss: repeatedly compressing and decompressing the file will cause it to progressively lose quality. This is in contrast with lossless data compression.
See more at Wikipedia.org...

...any process for compressing an image file that causes the file, once uncompressed to differ from the original appearance of the image file. Many lossy compression file formats look very similar to the uncompressed file, as the information that is discarded is often not easy to identify visually" (Vogt O'Connor 1999:49).