Lossy compression
A lossy data compression method is one where compressing a file and then decompressing it retrieves a file that may well be different to the original, but is "close enough" to be useful in some way.
The advantage of lossy methods over lossless methods is that in some cases a lossy method can produce a much smaller compressed file than any known lossless method, while still meeting the requirements of the application.
Lossy methods are most often used for compressing sound or images. In these cases, the retrieved file can be quite different to the original at the bit level while being indistinguishable to the human ear or eye for most practical purposes. Many methods focus on the idiosyncrasies of the human anatomy, taking into account, for example, that the human eye can see only certain frequencies of light. The psychoacoustic model describes how sound can be highly compressed without degrading the quality of the sound.
Lossy methods for still image compression:
Lossy methods for moving image compression:
Lossy methods for sound:
Both the popular, patented MP3 format and the free/free Ogg Vorbis format are lossy methods.
Lossy methods for anything else (anyone know any?):