Yes, there is a degradation in quality. Whether you can hear the difference depends on the equipment you're using to play the music and the environment in which it's played.
The music you buy from iTunes is in a compressed format that does not retain all the audio information available from the same music on a CD. If you want the highest quality you'll need to buy the music on a CD, DVD-A or SACD.
If you're listening on an mp3 player with earbuds on an airplane or subway you likely won't hear any difference. If you're playing the music through a reasonably good home system you'll definitely hear a difference.
The music you buy from iTunes is in a compressed format that does not retain all the audio information available from the same music on a CD. If you want the highest quality you'll need to buy the music on a CD, DVD-A or SACD.
If you're listening on an mp3 player with earbuds on an airplane or subway you likely won't hear any difference. If you're playing the music through a reasonably good home system you'll definitely hear a difference.