A lossy compression method is one where compressing data and then decompressing it retrieves data that is different from the original, but is close enough to be useful in some way.
Out of all the manipulations that can be carried on a sampled audio signal, compression in particular deserves a section of its own to take a look at its specific characteristics.
Lossy compression algorithms are based on the knowledge of the characteristics of the data to be compressed. Depending on what kind of data, it is possible to pin-point recurring parameters which can be used to carry out the compression.
Lossy audio formats comparison A comparison of lossy audio formats Ogg Vorbis, MP3, AAC, Musepack and WMA, looking at compressed file size and time taken to rip from CD.