Sound Check - Sound Check is a feature of iTunes, that can be used on the iPod, that normalizes the volume of the songs in your iTunes library.
Because songs are recorded at different volumes and with different technologies over time, the volume that they play back at can be different. Sound Check scans your iTunes library and attempts to bring the volume of all songs in it into rough equivalence. It does this by scanning the playback volume of songs in the library and adjusting the "normalization information" ID3 tag for the song.
Sound Check does not actually change the volume of the orignal song.
Sound Check can be used with iPod playback as well. As long as it is enabled in iTunes, the Sound Check-enhanced songs will transfer to the iPod and play at a roughly equal volume.