Depends on which iPod you're talking about.

Usually Apple changes few features and in some cases, design. A major difference with the iPod Classics being that with each generation, the sound processor was changed.

If you're talking about the Nano, the 5th generation has a built in microphone, camera and speaker, where the 4th generation does not. The prices for the larger storage iPod Nanos are also less than they were with the 4th generation versions. As far as I know, the audio processor did not change.

I hope that helped!