most of the tracks from iTunes are nowadays DRM free (ie no copy protection). Protected files are .m4p and DRM free are .m4a. there's also your normal mp3 files if you ripped tunes yourself and not have the convert option set in iTunes preferences.
Even the copyrighted files can be moved back and forth as many times as you like, if you use the same ID that you used when you bought the tunes.
If you copy copy protected tracks from someone else and try to play them in your iTunes, you'll get a prompt with your friends' email asking for the password. if you login using your friend's credentials, then you won't be able to access your purchased tracks.
if you want to convert copy protected tracks to non copy protected ones, just burn an audio CD with those tracks and then rip the tracks from the CD.