MPEG-2 Audio Layer III (MP3) is a lossy compression format for digital audio. It is the most popular format for digital music today. MP3 files can be encoded from CDs, but an increasingly common source is the internet. The rise of online music markets (as well as torrents and file sharing programs) has made obtaining MP3 files much easier and cheaper than in the past.
Popular MP3 programs on Ubuntu are Banshee, Rhythmbox and Clementine. These programs will usually support a variety of audio formats, and also have the ability to sync with MP3 players and other mobile devices capable of playing audio.