MP3
The MP3 format is a lossy compression format. This means that each time something is compressed with it, some information is lost. This information can not be recovered. When encoding to MP3 (converting some audio data to it), the encoder deletes some not important parts. The human ear can not hear certain sounds if they are masked by other sounds. This means that when encoding certain sound samples, some sounds can be left out as they will not be heard. This makes it possible to compress audio by a factor of about 5, without noticeable change to the sound of the music. With slight audible changes to the sound, compression factors of about 10 are possible.