MP3 and MP4

[Kiwi the 1st]

All the music I've downloaded from iTunes are MP4.

All the music I've put on iTunes from CD's are MP4.

My question is, why is it, that when you download music from either Limewire or Flickr Music, it's always MP3?

What's the difference between them?

Also, does it matter that on iTunes, I have some music that is MP3 or MP4?

Thanks.