A .m3u file is a playlist for multimedia files. It was originally implemented in Winamp but since has been increasing supported by other audio playback software such as Foobar 2000 and Windows Media Player.
It stores the location of one or more audio files. It can reference files stored in the same folder, on a separate folder on the same HDD or another or even a URL. It's simple a text file so if you would like to know more about how it works, open it with a text editor like notepad.
Related software tools
Foobar 2000 (Freeware)
Foobar is an advanced audio player for audiophiles
Winamp Full (Freeware)
Nullsoft Winamp is a fast, flexible, high-fidelity media player for Windows.