M3U was originally developed in late 1990s as a standard playlist format for MP3s, although its use has since then expanded beyond MP3s.
The .m3u files are basically just textfiles that list one MP3 or other media file on each line, normally with full path or URL to the file. If the .m3u file is loaded to an MP3 player, the player normally plays the list of media files in the order they are listed in the playlist (unless options such as "randomize" have been selected in the MP3 player).
