From Cow Pimp Productions:
iTunes is mainly meant to be used as a music manager and means of purchasing music online. However, in addition to purchasing music, one can listen to any of the music shared by others on a network. So, for example, if another person on your network has iTunes and decides to share their music, you can listen to it. iTunes does not allow you to save this music to your hard drive .
MyTunes adds this previously unimplemented feature by allowing you to save music from other computers to your hard drive.
Note: Some users have experienced problems when downloading many songs at once. In order to decrease the probability of something going wrong, it is suggested that you limit downloading to somewhere around 4 or 5 downloads at a time.