Has a very good explanation, and short history, of both MP3 and MP4.
The MP4 is only used through itunes and ipods, but the MP3 is a "standard" for most players and converters.
Fred was a little off when he said "MP4 is only used through itunes and ipods". That's the audio-only part of mp4 also known as AAC in the iTunes world. Mp4s are also related to video content as well: http://en.wikipedia.org/wiki/Mp4